Allied missiles slammed into a Baghdad bomb shelter early Wednesday, killing 500 civilians, Iraq said. The United States said the site was a bunker used as a military command center.

The number of deaths would be the single biggest loss of civilian life reported in the Persian Gulf war.The U.S. military command said the bunker had been painted in camouflage colors recently and was used by the Iraqi military command.

"We don't feel we attacked the wrong bunker or we made a mistake," Marine Brig. Gen. Richard I. Neal told reporters in Riyadh. "I can't explain if there were civilians in there why they were in there."

"The loss of civilian lives in time of war is a truly tragic consequence," White House spokesman Marlin Fitzwater told reporters in Washington.

The deaths occurred during a 12-hour bombardment of Baghdad. One of the buildings hit - the Palace of Conferences - is across the street from the al-Rashid Hotel, where a Soviet envoy on a peace mission was staying. The diplomat met Tuesday with Iraqi President Saddam Hussein.

As the war turned 4 weeks old, Soviet diplomatic efforts to end it picked up with the announcement of plans for Iraqi Foreign Minister Tariq Aziz to meet in Moscow with President Mikhail S. Gorbachev. Sunday's trip would be a follow-up to envoy Yevgeny Primakov's visit, the Kremlin said.

After meeting with Primakov, Saddam announced Iraq might be willing to talk. However, the White House noted he said nothing about the central issue in the conflict - ending Iraq's occupation of Kuwait.

At the United Nations, diplomats gathered Wednesday to consider whether the allies had exceeded the U.N. mandate against Iraq.

The air campaign continued at a relentless pace, with 2,800 sorties in the previous 24 hours, Neal said Wednesday.

During one mission over Iraq, a Saudi F-5 Tiger 2 was lost and the pilot was missing, Saudi Col. Ahmed al-Robayan said. It was the second Saudi plane lost.

Kuwait's air force used Gazelle helicopters Wednesday for the first time in the war, damaging two tanks, exiled Defense Minister Sheik Nawaf al-Ahmed said.

Despite the bombings, U.S. military officials said Iraq has managed to rig makeshift military communications and supply links. They said Iraq was drawing on battlefield experience from the 1980-88 Iran-Iraq war.

The reinforced structure hit in Baghdad Wednesday also was a legacy of that war: It was one of five big air-raid shelters built in the capital during that conflict. Neal said it had been upgraded to a hardened command and control bunker used for communications.

"If we thought that in fact there were civilians . . . absolutely no, we would not have attacked that bunker," Neal said.

"From a personal point of view, I'm outraged that civilians might have been placed in harm's way and I blame the Iraqi government and the Iraqi leadership for that," he said.

Iraqi witnesses said the shelter in the middle-class residential al-Amerieh district took direct hits from at least two missiles fired by allied warplanes. The missiles pierced 9 feet of concrete.

Rescuers clawing through the debris found eight survivors immediately after the bombing, AP correspondent Salah Nasrawi reported from the scene. The rescuers later said they retrieved more than 200 bodies, most of them charred and mutilated beyond recognition, Nasrawi reported.

Iraqi civil defense officials said lists compiled from residents indicated there were more than 500 people inside the structure when the warplanes struck before dawn. A senior civil defense official said scores of people remained buried in the rubble, but there was no hope of finding anyone alive.

Residents crowded around the wreckage, looking for relatives and friends. Men beat on their chests and yelled "Allahu Akbar," Arabic for "God is Great," and women cried hysterically, Nasrawi said.

At a Baghdad hospital where the injured were taken, 17-year-old Omar Adnan, badly burned, said he was the only survivor of his family of six.

"I was sleeping and suddenly I felt heat and the blanket was burning," Adnan said. "I turned to try and touch my mother who was next to me but grabbed nothing but a piece of flesh."

Also hit in the raids were telecommunications centers in the al-Jadrieh and al-Jamila districts, and the government's Palace of Conferences, Nasrawi said.Iraq previously had claimed thousands of civilian casualties in the allied air raids. U.S. officials have said the Iraqi figures were probably inflated.

President Bush, speaking with reporters in Washington on Tuesday, described Iraqi statements on civilian casualties as a "one-sided propaganda mission cranking out myths and falsehoods."

However, the U.S. command has said civilian casualties are an inevitable byproduct of the strikes aimed at military and strategic targets.

The White House also said Wednesday that Saddam is purposefully sacrificing Iraqi citizens.

In a statement before cameras, Fitzwater said: "The bunker that was attacked last night was a military target, a command and control center that fed instructions directly to the Iraqi war machine, painted and camouflaged to avoid detection, and well-documented as a military target."

He also restated that U.S. policy is that "we will not target civilian facilities" and that American bombers would "continue to hit only military targets."

There were other reports of civilian casualties Wednesday. Jordanian refugees said 30 people were killed and about two dozen hurt when allied planes attacked the bus they were riding in near the Iraq-Kuwait border on Saturday.

"We heard the planes come and they dropped bombs. One fell in front, one hit the bus and one fell behind, said Shehadeh Ibrahim Shehadeh, who arrived at the Jordanian border post of Ruweishid Wednesday. "The bus caught fire and some were trapped."

With the war thundering ahead, Baghdad radio reported early Wednesday that Saddam told Primakov he is prepared to cooperate with the Soviet Union and other nations in a peace effort.

But there was no sign Saddam is willing to relinquish Kuwait as the United States has demanded.

"The deciding factor for us is: What does he say about getting out of Kuwait?" Fitzwater said.

"Saddam Hussein created this war. He created the military bunkers and he can bring the war to an end. We urge him once again to save his people and comply with the U.N. resolutions."

At the United Nations, Cuba and Yemen were leading a push to keep the first Security Council debate on the war from being held in closed session. Both sides said Wednesday's vote could be close.

The closed session was sought by U.S. and British diplomats who maintain an open meeting with angry comments about the war would suggest the council did not stand by its resolutions to force Iraq to withdraw from Kuwait.

On Tuesday, Iraq said any ground war would be costly and bloody for the allies.

"If the Americans want to spare their sons from certain death, all they have to do is pull their losing forces out of the holy land of the Arabs and leave the Arabs to iron out their differences by themselves," Iraq's news agency quoted its parliament speaker, Saadi Mehdi Saleh, as saying.

But the allies signaled that it would be some time before they choose to wage an all-out ground war.

British Defense Secretary Tom King, who met with Bush in Washington on Tuesday, said there was "a need to see some significant reduction in Iraq's military capability" before any land assault.

"I think there's some work to be done," he said.