Insurgents fired a rocket at the headquarters of the U.S.-led coalition Monday night after gunmen killed seven Iraqi policemen in a pair of attacks west of Baghdad. A senior Iraqi official blamed Al Qaeda (search) for many of the homicide bombings around the country in recent weeks.

In the north, military divers searched the muddy waters of the Tigris River (search) for three missing U.S. soldiers, including two pilots of an OH-58D Kiowa Warrior (search) helicopter that crashed Sunday in Mosul during rescue operations after a patrol boat capsized.

It was the fifth U.S. helicopter lost in Iraq this month -- three from hostile fire.

Strong explosions reverberated through the heart of this troubled capital about 10:35 p.m., followed by sirens and a recorded message warning those in the coalition headquarters compound known as the green zone to "take cover."

A coalition official said at least one rocket, apparently fired from across the Tigris, exploded in a parking lot near the Republican Palace, used by America's top civilian administrator in Iraq, L. Paul Bremer, and senior coalition staff, but it caused no damage or casualties.

The attack occurred three days after a U.N. security assessment team arrived in Baghdad to determine whether it is safe for the United Nations to return its international staff to Iraq.

Secretary-General Kofi Annan withdrew international staff last year following two vehicle bombings against the U.N. headquarters here, including the Aug. 19 attack that killed 22 people, among them the top U.N. envoy, Sergio Vieira de Mello.

The attack on the green zone took place a day after the seven policemen were slain in two separate attacks at checkpoints around the city of Ramadi, 70 miles west of Baghdad in the Sunni Triangle, a center of resistance to the U.S.-run occupation. Iraqi police who reported the attacks made no mention of any insurgent casualties.

Attacks against U.S. forces and their Iraqi allies have persisted despite the Dec. 13 arrest of Saddam Hussein, who was captured near his hometown of Tikrit. Many of the victims have been Iraqi civilians.

One Iraqi man was killed Monday when he stepped on a roadside bomb as he got off a bus in a Baghdad suburb, Iraqi Civil Defense Corps 2nd Lt. Mustafa Tariq said. The explosion wounded three other passengers, one critically, and left the bus badly mangled, he said.

Late Monday, a roadside bomb exploded in a west Baghdad neighborhood, wounding one civilian and damaging at least three vehicles, witnesses said.

Elsewhere, two projectiles exploded for a second straight day Monday at the U.S. military base in Kirkuk, Iraqi police said, but there were no reports of damage or casualties. Witnesses also reported explosions near a Spanish garrison outside of Najaf, but the Spanish Defense Ministry said the base was not attacked.

Earlier Monday, the government minister in charge of internal security blamed Al Qaeda for some of the attacks, especially homicide car bombings throughout the country. These include the Aug. 19 truck bombing at the U.N. headquarters in Baghdad that killed 22 people, among them the top U.N. envoy, Sergio Vieira de Mello.

"There is a presence of Al Qaeda in this country," Interior Minister Nouri Badran said. "We've announced that directly and indirectly. A lot of the homicide attacks have the fingerprints of the crimes committed by Al Qaeda."

Badran offered no specific evidence to support his claim, and U.S. military officials have said they believe the overwhelming majority of attacks have been the work of Iraqis loyal to Saddam.

However, a U.S. official in Washington said Saturday that Kurdish forces had captured a senior Al Qaeda figure as he tried to enter northern Iraq. Hassan Ghul, a senior facilitator in Usama bin Laden's terror network, was turned over to the United States and is being interrogated at an undisclosed location, the official said on condition of anonymity.

On Monday, President Bush praised Ghul's capture, saying it was an example of "further progress in making America more secure." Bush told an audience in Little Rock, Ark., that Ghul reported directly to Khalid Shaikh Mohammed, who was captured last March near Islamabad, Pakistan.

"He was a killer," Bush said of Ghul. "He was moving money and messages around South Asia and the Middle East to other Al Qaeda leaders. He was a part of this network of haters that we're dismantling."

In Mosul, 225 miles northwest of Baghdad, divers and patrol boats searched the Tigris throughout the day for the three missing soldiers. One of the three disappeared when a patrol boat capsized Sunday, killing two Iraqi policemen and an Iraqi translator accompanying the American soldiers.

A Kiowa helicopter searching for the missing soldier crashed in the river hours later after witnesses said it struck a cable. The wreckage of the helicopter was recovered, but the two crew members were still unaccounted for.

The missing soldier from the boat is from the Stryker Brigade, which recently deployed to Iraq from Fort Lewis, Wash. The pilots are from the 3rd Squadron, 17th Cavalry Regiment, based at Fort Drum, N.Y.