The U.S. Central Command said Thursday there was "increasing evidence" that Saddam Hussein's regime had lost control of its fighting forces and most of the Iraqi population.

Brig. Gen. Vincent Brooks said coalition forces were approaching Baghdad, without giving a precise location. Reporters traveling with military units said leading elements were within 10 miles of the capital.

"We certainly are in close proximity to Baghdad," Brooks said.

"There is increasing evidence that the regime cannot control its forces or the Iraqi population in most of the country," he said.

In the first days of the war, which began March 20, U.S. commanders had frequently said the regime was losing its authority. Those assessments ranged from Iraq's command and control structure being "less robust" than it had been, to failing to see evidence that the regime was being controlled from the top.

Brooks said the U.S. military was investigating the loss of two of its aircraft on Wednesday -- a Navy F/A-18C Hornet fighter jet and an Army Black Hawk helicopter. Both were believed lost near Karbala, about 50 miles south of Baghdad, according to officials in Washington.

Asked whether the Hornet may have been hit by a U.S. Patriot missile, Brooks said there was missile fire in the area at the time, including both surface-to-air and surface-to-surface launches.

"It's too early for me to be able to determine what the cause was," he said.

A later Central Command statement said a U.S. Patriot missile may have downed the Hornet over central Iraq, but the circumstances remain under investigation.

Iraqi Information Minister Mohammed Saeed al-Sahhaf said an aircraft had been shot down by Saddam's Fedayeen forces.

Pentagon officials initially said the Black Hawk was downed by small arms fire near Karbala. But Brooks said: "We don't think it was a result of hostile fire."

Brooks said there were "some casualties" from the helicopter crash. The Pentagon said six Americans aboard the helicopter were killed. The Hornet pilot was missing, and search and rescue operations were under way, officials said.

A key question was the fighting strength of the Republican Guard units stationed around Baghdad, which U.S. officials have said were hit hard.

Brooks said some Republican Guard had surrendered, including 53 who gave up in the town of Kut, about 80 miles southeast of Baghdad. He said a number of guard units were on the move, but it was unclear whether they intended to fight or run.

"Whether (the Republican Guard) is melting away by deciding not to fight anymore, or whether it is repositioning, there is movement," he said.

Brooks said even though U.S. forces were near Baghdad, "we don't think that the fighting is over yet."

"There are still options that are open to the regime, including weapons of mass destruction. We take that very seriously," he said.

Brooks said special operations forces had seized the Thar Thar presidential palace about 56 miles northwest of Baghdad. Earlier, a Central Command spokesman said the palace had been in the vicinity of Baghdad and near the airport, south of the capital.

The palace has been used as a vacation retreat for Saddam and his sons and is located on a lake that is a favorite fishing spot for the Iraqi leader. Brooks said no regime leaders were present when U.S. forces raided the building.

During the raid, Special Operations forces encountered sporadic and light resistance from fighters on the ground, but troops overwhelmed them, said Lt. Mark Kitchens, a Central Command spokesman.

There was no indication Saddam had been to the palace recently, Kitchens said.

Brooks said important documents had been seized from the palace, but he provided no details.

Brooks said that Ayatollah Sayyid Ali al-Husseini al-Sistani, a Shiite leader who had been under house arrest in Iraq, had issued a religious ruling called a fatwa urging Iraqis not to interfere with coalition troops and to remain calm.

He called it a "courageous statement."

"We are seeing evidence of other religious leaders who have had enough of this regime," he said.

Iraq said al-Sistani issued no such call.

Al-Sistani is considered by many Shiites as grand ayatollah, or supreme spiritual leader. He has millions of followers inside and outside Iraq.

Sistani succeeded the late Ayatollah Abu al-Qassim al-Khoei as a spiritual leader after Khoei's death in 1993.

On Sept. 22, he reportedly issued a fatwa urging Muslims to defend Iraq in the face of a potential U.S. military attack.

Iraqi Information Minister Mohammed Saeed al-Sahhaf told Al-Jazeera television on Thursday that Shiite Muslims did not issue a fatwa not to fight coalition forces, adding that "all their fatwas call for fighting the American aggressors."