U.S. forces closed in on Saddam Hussein's center of power Wednesday, forcing Iraqis to move more elite Republican Guard troops to the south amid fighting that downed two American aircraft.

The movement of Iraqi units to face the American advance came as elements of the Army's 3rd Infantry Division closed to within 20 miles of Baghdad's city limits.

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

Meanwhile, Iraqis shot down an Army Black Hawk helicopter and a Navy F/A-18C Hornet. Both aircraft went down near Karbala, about 50 miles south of Baghdad, where U.S. Army units had battled Republican Guard units.

Pentagon officials said seven soldiers on the helicopter were killed, and there was no word on the fate of the Navy pilot.

The Black Hawk came down under small-arms fire, defense officials said. Four other soldiers on the helicopter were wounded and rescued after the crash.

The Army forces had surrounded Karbala, a holy site to Shiite Muslims, on Tuesday as they broke through Republican Guard units and pushed toward Baghdad.

The twin-engine Hornet, flying from the aircraft carrier USS Kitty Hawk, was downed by an Iraqi anti-aircraft missile at about 3:45 p.m. EST Wednesday, military officials said.

The U.S. hopes to increase psychological pressure on Saddam and his inner circle as American-led forces arrive on the outskirts of Baghdad, the military official said.

Still, the Pentagon sought Wednesday to lower expectations that the Iraqi capital could be taken quickly or easily.

"We are planning for a very difficult fight ahead in Baghdad," Maj. Gen. Stanley McChrystal told a Pentagon news conference. "We are not expecting to drive into Baghdad suddenly and seize it."

Defense officials, updating the war toll, said 49 Americans had died, seven have been captured, 15 are missing and 154 have been wounded.

U.S. military officials have weathered criticism over the past week as commanders, other Pentagon officials and outside analysts said the war effort had been slowed by unexpectedly stiff resistance and a plan that may have relied on too few troops.

McChrystal and department spokeswoman Victoria Clarke told reporters at least six times in a half-hour briefing that the toughest fighting may lie ahead.

McChrystal said a major American offensive had pushed closer to Baghdad amid resistance from Saddam Hussein's Republican Guard that was "sporadic, but not able to stop coalition maneuvers."

Republican Guard forces, better trained and equipped than Iraq's regular army, were "arrayed for a defense on the southern side of Baghdad ... and on the flanks as well," McChrystal said. "Whether they intend to defend in place or just delay is just not clear."

It was unclear whether those Iraqi forces had some of the chemical and biological weapons that U.S. leaders say Saddam is hiding -- and whether they could or would use them.

From the start of the war, the Pentagon has hoped that with the help of invading forces the Iraqi military would surrender in great numbers and civilians might revolt.

With ground forces pressing closer to Baghdad, officials said they were hopeful that the chances of those things happening were increasing.

The assessment came after a daylong battle against Guard forces on the outskirts of the capital.

Though two divisions were reported to have been largely eliminated as effective fighting forces, there were four others that officials said remained somewhat intact. Two were estimated at 70 percent effectiveness and two somewhat less, according to a defense official who spoke on condition of anonymity.

Other officials said some of the Republican Guard units had started mixing together, making it nearly impossible to state where each of the divisions was based.

The last line of defense for Baghdad may be Saddam's Special Republican Guard and Special Security Organization. It was not clear what damage had been done to the Special Republican Guard, for instance, a light infantry force that was estimated to have started the war with 15,000 troops. The security organization is basically a paramilitary force estimated at 6,000 to 8,000.