Flush from the taking of Baghdad and with the toppling of Saddam Hussein's statue still fresh on their minds, coalition forces on Thursday focused their attention on the northern Iraqi cities of Tikrit, Mosul and Kirkuk, where they will try to stamp out the remaining fighters loyal to Saddam.

In Baghdad, four U.S. Marines were seriously wounded in an apparent suicide bombing attack, Fox News confirmed. The attacker reportedly was killed. A U.S. officer reported "some dead" at the scene, but it was not immediately clear if any Americans had been killed.

• Maps: Iraq | Baghdad

In the latest sign that Saddam's stronghold is crumbling in the north, America's Kurdish allies entered Kirkuk near some of Iraq's most productive oil fields with little to no resistance, essentially seizing the city.

A convoy of Kurdish fighters driving into the city in about 100 vehicles triggered celebrations by Kurdish residents, looting of government buildings and the toppling of a statue of Saddam in a central square.

Turkey is upset that the Kurdish fighters are in the region, and is sending military observers to Kirkuk. The Turkish prime minister said Secretary of State Colin Powell assured him the Kurds would be removed from the city.

"We've been in contact with officials in Turkey as well as free Iraqis in the north and I think it is fair to say that American forces will be in control of Kirkuk," White House spokesman Ari Fleischer told reporters.

The United States has opposed Turkey sending troops to the region out of fear their presence would inflame tensions with the Kurds.

Meanwhile, there have been reports that Republican Guard troops have been funneling into Tikrit and Mosul.

Senior defense officials told Fox News Thursday that "no command and control is being detected at all in Baghdad," but that some command and control functions remained near Tikrit, Saddam's ancestral homeland.

The officials also identified Izzat Ibrahim, vice chairman of the Revolutionary Command Council and deputy commander of the Iraqi armed forces, as the leading figure trying to mount a defense in Tikrit.

The officials said "elements" of up to eight regular Iraqi army divisions remain in the north -- but these forces are not expected to mount a serious defense.

Officials also said "parts of one brigade of a Republican Guard division" remain and are expected to join one brigade of the Special Republican Guard to defend Tikrit.

The Dubai-based Arabic television network al-Arabiya said U.S. air strikes were hitting the frontlines in Mosul.

In the southern city of Najaf, a crowd hacked to death a senior Shiite Islamic leader -- Abdul Majid al-Khoei -- and a Saddam Hussein loyalist -- Haider Al-Kadar -- at a meeting meant to forge reconciliation in one of Shiite Islam's holiest shrines, witnesses said.

In Baghdad, meanwhile, skirmishes continued to break out between U.S. forces and Iraqi holdouts as looters took to the streets for the second day.

At least two explosions rocked the southern end of the Old Palace presidential compound Thursday evening, starting several small fires. U.S. Army troops occupying the compound appeared to return fire with tank cannons.

"Despite what you see in terms of localized euphoria ... this operation is a long way from complete," Air Force Maj. Gen. Victor Renuart said during a U.S. Central Command briefing in Doha, Qatar.

"There's a long way to go still … Baghdad's still an ugly place."

Battle at the Mosque; Bush addresses Iraq

Iraqi fighters in northern Baghdad's Imam Mosque opened fire Thursday on U.S. Marines who were hunting for regime leaders, triggering a fierce battle.

Marines from the 1st Marine Expeditionary Force were scouring the area after U.S. officers received a tip that regime leaders were trying to organize a meeting there, at the house of a senior Baath Party official. Military officials don't know if Saddam was among the leaders there.

The "intense fighting" took place near the Az Amihyah Palace. Central Command spokesman Frank Thorp said. One Marine was killed and up to 20 others were wounded in fighting at the palace. Thorp said the mosque was in good condition.

"The regime of Saddam Hussein is being removed from power" and freedom will soon reign, President Bush told Iraqis in televised remarks broadcast throughout Iraq Thursday.

The address was broadcast by the U.S. military as part of an information program televised to the Iraqi people.

"The long era of fear and cruelty is ending," Bush said. "The government of Iraq and the future of your country will soon belong to you."

Motorists flew white flags on their vehicles; many people made off with carpets, furniture and air conditioners from government-owned apartments, abandoned government ministries and the German Embassy.

"I'm 49, but I never lived a single day. Only now will I start living," said Yussuf Abed Kazim, a mosque preacher.

A young Iraqi spat on a Saddam portrait. Men hugged Americans in full combat gear; women held up babies so soldiers riding on tanks could kiss them.

The BBC reported on its Web site that U.S. Marines had also stormed a Baghdad mosque; there was some belief that the Saddam might be hiding inside.

Saddam Loyalists May Be Heading for Syria

Defense Secretary Donald Rumsfeld said some unidentified members of Saddam's regime were moving out of Iraq into Syria.

U.S. officials said Thursday that U.S. forces are gaining control of Qaim on a key road at Iraq's border with Syria.

To the south, officials said the 1st Battalion Royal Irish Regiment had reached Qurnah, said to be the site of the biblical Garden of Eden.

The urgent need for humanitarian aid became heightened Thursday, as civilians struggled with shortages of food, medicine and clean water. International aid groups are demanding swift access to civilians.

Headed by retired U.S. Gen. Jay Garner, the U.S.-led interim administration team will coordinate relief programs, rebuild shattered infrastructure and start setting up a democratic government.

In response to rampant looting, some senior Iraqi clerics are asking coalition forces to reinforce curfews and deem looting illegal.

British troops on Thursday asked Basra residents to turn in their guns.

"Iraq has a culture of weapons," said Capt. Cliff Dare of 3 Commando Brigade Engineer Group. "If we want to give the new Iraq a chance, these weapons have to be taken out of circulation."

The Search for WMDs

U.S. officials told Fox News that CIA officers are in Baghdad and throughout Iraq looking for scientists to point out the locations of Iraq's weapons of mass destruction. The intelligence operation was described as "extensive."

Special operations teams are also trying to find and test possible weapons materials at 1,000 sites. Senior defense officials said, "The list has grown ... and [the teams] have only been through a small number of sites (fewer than 20)."

At one site, Marines discovered an enormous cache of weapons, including hundreds of mortars and grenade launchers, and millions of rounds of ammunition.

Senior defense officials also told Fox News that one MOAB bomb -- the "Massive Ordnance Air Blast" munition nicknamed the "Mother of All Bombs," has been moved into the Iraqi theater, and is ready for use. The bomb is being kept on hand as a contingency.

"I can't think of a situation in which we would use one," said one senior U.S. official.

Rumsfeld said several missions remain to be accomplished: securing the northern oil fields, determining what happened to Saddam and his sons, uncovering details of Saddam's weapons programs, and capturing or killing any terrorists still at large in Iraq.

Saddam's whereabouts since Monday's bombing remained a mystery. U.S. special operations forces scoured the site where he may have been killed Wednesday.

Fox News' Bret Baier, Major Garrett, David Lee Miller, Eric Shawn and Liza Porteus and The Associated Press contributed to this report.