Acting quickly on newly obtained intelligence, a U.S. warplane bombed a downtown Baghdad building yesterday where Saddam Hussein, his two sons and senior Iraqi intelligence officials were believed to be meeting, U.S. officials said.

Although it was not immediately clear who was in the building and whether anyone survived, U.S. officials were "moderately hopeful" that the Iraqi president was killed, a senior U.S. government official said.

Other officials expressed more caution. "There was some credible intelligence that there were senior people there," one said. But, he added, "we don't know if Saddam Hussein was there."

The airstrike was carried out by a single B-1 bomber that dropped bunker-penetrating bombs on the building. According to U.S. officials, intelligence authorities learned of the meeting's location yesterday morning and relayed the information to the military's Central Command in Qatar, which already had the bomber in the air.

The plane "fairly quickly" dropped four warheads on the target, before anyone had left the building, officials said.

Marine Capt. Stewart Upton, a spokesman for the U.S. Central Command, confirmed the attack, saying that Joint Direct Attack Munitions were dropped on a site in Baghdad's Mansur district at 2 p.m. local time. "It was a leadership target, and it was hit very hard," he said.

He added: "We cannot confirm that Saddam Hussein is dead."

Mansur is an upscale residential neighborhood in west-central Baghdad.

U.S. ground forces, which have been operating inside Baghdad for more than a day, may be able to secure the area and inspect the damage, something that has not been possible before.

While targets associated with Hussein and other top members of the Iraqi government have been hit routinely, yesterday's airstrike was unusual. It was similar to the opening volley of the war March 20, when President Bush and his top advisers acted on perishable intelligence about Hussein's whereabouts and attempted to kill him and his sons.

In that strike, F-117A stealth fighters dropped a pair of 2,000-pound bombs on a bunker in southern Baghdad where Hussein and his sons were believed to be spending the night. The bombs were quickly followed by a volley of Tomahawk cruise missiles from U.S. warships, severely damaging the complex, known as Dora Farm.

Some U.S. intelligence officials believed that Hussein had been injured or killed in that attack, and they debated whether his subsequent television appearances had been taped in advance. But they gradually concluded that he had survived.

Even before yesterday's airstrike, Hussein's government appeared to have only feeble control over disjointed elements of Baghdad-based security forces and virtually no significant control over conventional military units in the capital or elsewhere in the country, U.S. military, intelligence and administration officials said yesterday.

Officials cautioned that the war is far from over, and that allied forces are now facing combat against tough Iraqi paramilitary units hidden in Baghdad, a city of 5 million. Still, there is increased confidence that the Iraqi government can no longer coordinate or mount sizable attacks of military or paramilitary units.

"There seems to be a few vital signs left," one senior U.S. government official said, "but the regime is increasingly looking like it's brain-dead."

Buoying U.S. officials was the reported death in an allied air attack of Hussein's cousin Ali Hassan Majeed, better known as "Chemical Ali" for his use of chemical poisons against the Kurds in the late 1980s. Also apparently killed in the attack was the director of a feared Iraqi intelligence service, the Mukhabarat. Tahir Jalil Haboush was reportedly with Majeed at a base outside Basra when it was struck by warplanes. "We believe that the reign of terror of Chemical Ali has come to an end," Defense Secretary Donald H. Rumsfeld told reporters yesterday. "To Iraqis who have suffered at his hand . . . he will never again terrorize you or your families."

The most worrisome paramilitary units -- Saddam's Fedayeen and the Special Security Organization -- appear to be in control of only small sections of Baghdad, military and intelligence officials said. But it is not at all clear if they are working with a central authority or simply "out to save their own necks" from an Iraqi population that is expected to take swift and bloody revenge at the right moment, a senior administration official said.

As for the conventional military units, "command and control of the Republican Guard is at the point now where the worst thing they can do is sporadic attacks from very, very small units," Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, told reporters yesterday.

Even the Special Republican Guard and the Special Security Organization -- loyal, highly motivated forces -- do "not appear very coherent at this point. That does not mean that there's not tough fighting, that we maybe have not gotten to the point where we're going to see some of those units that I just mentioned become tougher adversaries," Myers said. "That's quite likely that that would happen."

Military officials said allied forces have destroyed all but about 18 of the Republican Guard's 800 tanks, and all but about 50 of its 550 artillery pieces. "We don't even think the Republican Guard can muster a battalion-level force" of several hundred men, they said.

Some northern units ordered south to Baghdad did not follow orders, either because they did not receive them, or because they refused, one U.S. intelligence official said. After heavy bombing of communication nodes, the only effective means the Iraqi government has left for communicating is satellite phones, which U.S. intelligence systems can monitor.

U.S. military and intelligence officials also have picked up Baghdad-based commanders giving orders to units that no longer exist, another government source said.

In addition to the U.S. conventional forces on the ground in Baghdad, covert Special Operations units and CIA paramilitary teams continue to operate in the city and other urban areas. Their focus, one longtime intelligence officer said, is to find Hussein, kill or capture senior government leaders and guide U.S. aircraft to Iraqi targets using hand-held laser devices.

As the result of these two parallel wars -- one highly visible in the daily rumble of tanks and bombs, the other invisible and covert -- the Iraqi government "hasn't fallen, but its grip is severely hampered, its command and control severely damaged," said the senior administration official, who has access to daily intelligence reports.

In a small but significant sign of their confidence, Pentagon, administration and intelligence officials said yesterday that the continued news conferences of Iraqi Information Minister Mohammed Saeed Sahhaf were not worrisome enough to stop. "He's not a very valuable target," one official said.

As Sahhaf spoke yesterday, a U.S. shell landed nearby. "Be assured Baghdad is safe, secure and great," the information minister said. "There is no presence of the American columns in the city of Baghdad, none at all."

As for Hussein, a recent intelligence assessment concluded that a videotape released last week of the Iraqi leader strolling among a cheering crowd in Baghdad was actually recorded long before the war began March 20. Among the indications that the tape was old, a defense official said, were the lack of damage in the area from recent bombings, and the weather conditions.

In addition, "there's always been the belief that when you see Saddam go into a crowd, it's always a double," an intelligence official said.

Bush administration officials have recently discussed a "tipping point" at which the destruction of Hussein's government and military is sufficient to convince Iraqi citizens that they can safely revolt against the remnants and bring the government down once and for all.

"I can't say we're at a tipping point," Rumsfeld told reporters yesterday. Instead, individuals, groups, villages and military units, one by one, may decide to move against the government, he said.

Rumsfeld said a country-wide, instantaneous revolt is unlikely to occur because "the facts on the ground are so different in different parts of the country. . . . I do think the concept of a tipping point is correct, and at some point, the aggregation of all of those individual tipping points having been reached, it will be in effect the country will have tipped. But it will be cumulative rather than at one moment."

Staff writer Alan Cooperman contributed to this report.

Air Force Lt. John Blocher reloads his Iraqi AK-47 rifle during an Army offensive in northern Baghdad. Fires burn in the background from destroyed Iraqi military equipment.U.S. Marines from the 3rd Battalion yell for infantrymen to rush across a damaged bridge yesterday as they move into Baghdad while under fire. The government of Saddam Hussein has all but lost control of the city.An Iraqi police officer voices support for Saddam Hussein as U.S. forces raid the presidential compound in Baghdad. Iraqi police officers wave their guns as they parade in front of the Palestine Hotel in Baghdad, where U.S. troops seized key buildings yesterday."Be assured Baghdad is safe, secure and great," Iraqi Information Minister Mohammed Saeed Sahhaf told reporters in front of the Palestine Hotel in Baghdad.