BAGHDAD, Iraq - Four US Marines died when a Sea Knight helicopter plunged into a lake in volatile Anbar province, the military said Monday, raising to 13 the number of American troops killed during a bloody weekend in Iraq.

In this photo released by the US Marine Corps, US Marines with Combat Logistics Battalion Helicopter Support Team hook a M198 Medium Howitzer to an approaching CH-53 E helicopter, in Al Asad, Iraq, Wednesday, Nov. 29, 2006. [AP]

It was the second military aircraft to go down in a week in Anbar, a stronghold of Sunni insurgents, although the military said mechanical problems rather than gunfire had forced the emergency landing on Sunday.

"The pilots maintained control of the aircraft the entire time," the military said.

A Marine was pulled from the water but attempts to resuscitate him were unsuccessful. The bodies of three missing Marines were found in a subsequent search, the military said. Twelve other passengers survived.

Lt. Col. Christopher Garver, a US military spokesman, declined to provide details about the twin-rotor CH-46 helicopter's mission or the reason for its forced landing, saying the incident was under investigation.

The helicopter, from the Third Marine Aircraft wing, had the ability to land and taxi in the water in case of emergency. It came down in Lake Qadisiyah, a huge reservoir behind the hydroelectric dam at Haditha on the Euphrates River.

The deaths came on a weekend in which nine other US troops were killed, including five in Anbar. The weekend's violence pushed the total number of American service members who have died since the war started in March 2003 to at least 2,901.

A US fighter jet also crashed last week in a field, killing the Air Force pilot.

Iraqi state TV, meanwhile, reported that Iraqi police found half a ton of explosives, including suicide belts and roadside bombs, in Anbar - a province the size of North Carolina that stretches west from Baghdad to the borders of Syria, Jordan and Saudi Arabia.

High numbers of US casualties as well as a recent spike in violence between Shiites and Sunnis have contributed to doubts about the war in the US These concerns were considered a major factor in the US Democratic congressional victory.

President Bush told one of Iraq's leading Shiite politicians, Abdul-Aziz al-Hakim, in a White House meeting Monday that the United States was not satisfied with progress in Iraq.

"I assured him that the US supports his work and the work of the prime minister to unify the country," Bush said, referring to Iraqi Prime Minister Nouri al-Maliki. "Part of unifying Iraq is for the elected leaders and society leaders to reject the extremists that are trying to stop the advance of this young democracy."

The president is under pressure to draft a new blueprint for US involvement in Iraq. A bipartisan commission - headed by James A. Baker III, former Republican secretary of state, and former Democratic Rep. Lee Hamilton of Indiana - is expected to present its recommendations to Bush on Wednesday.

The group is expected to recommend gradually changing the mission of US troops in Iraq from combat to training and supporting Iraqi units, with a goal of withdrawing American combat troops by early 2008.

The violence persisted Monday, with at least 13 people killed in attacks nationwide. The victims included Nabil Ibrahim al-Dulaimi, a 36-year-old Sunni news editor with the private, independent Dijlah radio station who was gunned down in his car on his way to work.

Al-Dulaimi's slaying raised to at least 93 the number of journalists killed in Iraq since the Iraq war began, according the Paris-based Reporters Without Borders.

Police also found 56 bodies in Baghdad and the province of Diyala, northeast of the capital. Forty-eight of those were handcuffed, blindfolded and shot before they were dumped in two different areas of the capital - 18 on the Sunni-dominated western bank of the Tigris River and 30 on the eastern side, which is largely Shiite.

The rampant sectarian violence led UN Secretary-General Kofi Annan to declare on Sunday that Iraq is suffering a civil war even deadlier than the one that decimated Lebanon in 1975-1990. Al-Maliki's office rejected the characterization.

America's top two officials in Iraq - Zalmay Khalilzad and US Gen. George W. Casey, Jr., the top American military commander in Iraq - issued a statement denouncing the surge of violence in the capital.

"We condemn in the strongest language the recent car bombings, attacks and retribution killings by extremists against peaceful Iraqis in Baghdad," they said. "We implore all Iraqis not to become pawns of those who seek to destroy you and your country. Do not allow yourself to be drawn down the road of senseless brutality by striking back."

In Washington, Bush and al-Hakim spoke for more than an hour, four days after the president met with the Iraqi prime minister in Jordan.

Shiite leaders fear that any solution formulated outside the Shiite-dominated government to compromise with Iraq's Sunnis.

"Iraq should be in a position to solve Iraq's problems," al-Hakim said.

In northern Baghdad, American forces killed two insurgents and detained six during a raid on buildings where insurgents with ties to al-Qaida in Iraq were making car bombs, the US command said. A weapons cache including artillery rounds and AK-47s also was found.

In northern Iraq, near the refinery city of Beiji, an Iraqi soldier opened fire on protesters who tried to break through a coalition checkpoint Monday. Three protesters were wounded, said Iraqi army Lt. Hassan Mohammed.

The shooting occurred after several hundred protesters marched from Beiji to the checkpoint in nearby Siniyah village, where a convoy of trucks carrying food apparently had been stopped. As a US military helicopter circled overhead, shots rang out and the demonstrators ran away.