As ground fighting intensified on the road to Baghdad early Thursday, a U.S. Army helicopter was shot down in southern Iraq, killing seven of the 11 soldiers onboard.

Pentagon officials said the chopper was downed by small-arms fire near Karbala, the site of fierce fighting between the Army's 3rd Infantry Division and Iraqi troops, including Republican Guard forces.

The other four soldiers on the Black Hawk were wounded and rescued, officials said.

• Map: The War in Iraq

The Black Hawk was the second U.S. helicopter to go down in combat. An Army Apache assault helicopter went down March 24 during an assault on Republican Guard forces; its two pilots were captured by Iraqis.

The UH-60 Black Hawk is one of the Army's main utility and troop transport helicopters.

Meanwhile, ground forces closed in on the Iraqi capital, with some American units reportedly just 15 miles away and others about 30 miles out. U.S. forces wiped out one Republican Guard division and nearly destroyed another as they geared up for an all-out assault on the Iraqi capital.

"Our guys are able to see the skyline. That's how close we've gotten," a senior military official said, speaking on condition of anonymity.

Also Thursday morning, a huge explosion was reported again in the capital; a pair of explosions was heard by witnesses after nightfall Wednesday.

Thousands of Marines who had protected the U.S. Army's eastern flank along the Euphrates River turned sharply east, joining other Marine forces moving quickly along the Tigris River southeast of Baghdad.

At the same time, Army troops southwest of Baghdad moved to within 20 miles of the city, said a senior military official in Washington.

Along the way, some Iraqi soldiers shed their uniforms for plain robes, and smiling civilians proffered the troops Iraqi-brand cigarettes.

"It feels better going north," said Marine Cpl. John Edwards of Clovis, N.M. "The sooner we do it, the sooner we go home."

The Baghdad division of Saddam Hussein's vaunted Republican Guard has been wiped out, and senior Defense officials told Fox News that the Medina Division was "almost completely destroyed as well."

"The Medina and Baghdad divisions are no longer credible forces," Maj. Gen. Stanley McChrystal said at the Pentagon. "There's clearly command and control evident," but "effective command and control and effective maneuvers are not as evident."

Food, fuel and medicine for exhausted U.S. troops arrived in northern Iraq from Turkey.

The Arab satellite channel Al-Jazeera announced early Thursday that Iraq was expelling one of its reporters from Baghdad and barring another from reporting. In protest, the station suspended "the work of all its correspondents in Baghdad, Basra and Mosul, while maintaining the broadcasting of live and recorded images received from its office in Baghdad," an Al-Jazeera statement said.

Late Wednesday night, rescued American prisoner of war Jessica Lynch, a 19-year-old Army supply clerk, arrived at a U.S. air base in southwestern Germany on a C-17 transport plane for treatment at a U.S. military medical center. Her condition was not disclosed, but U.S. officials in Kuwait said she was believed to have broken legs, a broken arm and at least one gunshot wound.

The Defense Department announced that 11 bodies were found during the special forces rescue operation in Nasiriyah. DNA tests are being conducted to determine if any were Americans.

Meanwhile, ground forces from the Army's 3rd Infantry Division and the First Marine Expeditionary Corps penetrated the Iraqi capital's "red zone," destroying the Baghdad division as they crossed the Tigris River. The "red zone" is where U.S. officials say Iraqi resistance will be most tenacious, and where Saddam has reportedly authorized his troops to use chemical weapons.

Lead U.S. infantry units donned their chemical suits after capturing a bridge 40 miles southwest of Baghdad. Some Marines began adding their protective boots to the suits they already wear, and Marine helicopter pilots were advised for the first time to be ready to don chemical suits at a moment's notice.

U.S. officials warned that a cornered Saddam might resort to unleashing his worst weapons. "There may be a trigger line where the regime deems [a] sufficient threat to use weapons of mass destruction," said U.S. Brig. Gen. Vincent Brooks.

McChrystal warned that it would be a "grave mistake" for Iraqi troops to use chemical weapons.

Evidence was found in the Kurdish-controlled regions of northern Iraq that the Islamic militant group Ansar Al Islam was working on three types of chlorine gas and ricin and has ties to Al Qaeda, U.S. officials told Fox News.

Officials said that between 75 and 150 Al Qaeda members have been captured or killed in northern Iraq in recent days.

U.S. sources told Fox News that documents and equipment were found in the rubble of an Ansar facility that had been built into a cliff near Sargat. The material was described as "a cookbook and kitchen" for chemical weapons. Other items included latex gloves, ampules of penicillin, a freezer and lab equipment. Sources said additional tests are planned.

Two suspected Al Qaeda members escaped into Iran, according to officials, but surrendered to the Iranian Revolutionary Guards. Iran has said it will hand over captured Ansar members.

'The Dagger Is Clearly Pointed'

The battering of the Republican Guard's Medina division is significant because that unit is an armored division; the Baghdad division didn't have such armor. One senior official described this as "the first step in taking down defenses outside Baghdad."

"We managed to take the divisions south of Baghdad from 50 percent to 20 percent of their original strength," the official said, adding that happened before the attack on the Medina division.

Still posing a threat is the Adnan division, a mechanized armor division, which has moved to the west side of Baghdad. Adnan is still believed to be above 50 percent of its original strength, but coalition airstrikes took out a brigade of its armor Tuesday night.

Another division, the armored al Nida division, which is located southeast of Baghdad, also is getting "hit hard … a heavy pounding," the official said. U.S. Marines heading north could encounter al Nida fighters, but U.S. officials believe their constant air campaign should further soften the resistance.

The Baghdad Division was destroyed while trying to guard the city of Kut.

"They're in trouble … they're under serious attack right now," Brooks said during a U.S. Central Command briefing in Doha, Qatar.

"The dagger is clearly pointed at the heart of the regime right now and it will remain there until it's [the regime] gone," Brooks said. "That dagger does remain pointed and it remains further in our control."

En route to Baghdad, U.S. forces seized a bridge over the Tigris and swept past battered Republican Guard units in Karbala. Marine helicopter pilots were advised to be ready to don chemical suits quickly if needed after crossing the Tigris.

The Army's 3rd Infantry Division seized a bridge over the Euphrates River at Mussayib, about 40 miles southeast of Baghdad, as it advanced through the Karbala Gap. The bridge, taken with little or no resistance from Iraqi forces, had been rigged with explosives, but engineers defused them.

American troops took a key bridge in the town of Numaniyah without a fight. Many Iraqi soldiers surrendered and others traded their Iraqi army uniforms for civilian clothes. U.S. Marines recovered Iraqi gas masks, mortar ammunition and rifles.

U.S. B-52s dropped six 1,000-pound cluster bombs on an Iraqi tank column moving toward American troops in central Iraq. It marked the first time in combat history that the armor-busting, sensor-fused CBU-105 Wind Corrected Munitions Dispensers were used in an attack.

"This is certainly a decisive engagement in which we are now just beginning with the Republican Guard," said British Air Marshal Brian Burridge.

Working Toward a Synchronized Attack

Eventually, the U.S.-led forces intend to launch a synchronized attack on Baghdad with the infantry, the Marines and the Air Force.

"Synchronicity is a very strong doctrine of the United States Army," said Central Command spokesman Navy Capt. Frank Thorp. "To synchronize the different battles on the field provides that tactical surprise."

Iraqi Information Minister Mohammed al-Sahhaf said reports of a coalition breakthrough on that front are "illusions."

Brooks said two other Republican Guard divisions were engaged around Karbala and that coalition forces had seized control of a dam on Lake al-Milh.

U.S. forces also fought the Republican Guard's Medina and Nebuchadnezzar divisions, and attacked positions north of Karbala, where 2,000 fedayeen loyalists and Baath Party members were reportedly hunkered down.

At least 20 Iraqis were reported killed and an unknown number taken prisoner.

"The battle is being waged and we are doing well," Thorp said. "We have moved beyond where the Republican Guard is and beyond where the popularly known red line is."

Farther south, the northbound highway out of Nasiriyah was full of coalition military convoys. Marines staged a nighttime raid there earlier and found Iraqis had abandoned a huge, walled police compound.

"The noose is starting to tighten around Baghdad," said Sgt. Jeff Lanter.

Coalition officials said more Iraqi civilians are helping allied forces.

An "increasing numbers of the Iraqi people are aware of what's going on," Clark said. "I think they're getting a better sense that this regime is coming closer to an end."

POW Rescued

Lynch was freed after nine days in Iraqi hands when U.S. commandoes stormed the hospital in Nasiriyah where she was being held. She was listed as missing March 23 along with 11 other U.S. soldiers from the 507th Maintenance Company, based at Fort Bliss, Texas, after an ambush near Nasiriyah.

Following an intelligence tip about Lynch's whereabouts, U.S. special operations forces made their way behind Iraqi lines and seized Lynch from the Saddam Hospital under cover of darkness late Tuesday, American officials said.

Two of the 11 bodies found in the hospital during the rescue were in a morgue, while the nine others were in a grave area in the community, Brooks said. U.S. forces were led to the graves by someone taken into custody.

Brooks said ammunition, mortars, maps and a terrain model were found at the hospital, along with "other things that made it very clear it was being used as a military command post."

Separately, the Navy said two pilots were rescued after their F-14 Tomcat crashed in southern Iraq.

The International Red Cross said some its staff members saw the bodies of dozens of people at a town south of Baghdad where Iraqi officials claim U.S. helicopters attacked a residential neighborhood. At least 280 injured people were being treated at a hospital in Hillah.

Central Command is looking into an allegation that coalition aircraft mistakenly bombed a Red Crescent maternity hospital in Baghdad.

Fox News' Bret Baier, Carl Cameron, Major Garrett and The Associated Press contributed to this report.