BAGHDAD – A helicopter went down in northern Iraq today, killing all American soldiers aboard, the military said, the deadliest crash since January 2005.

The military said initial indications showed the aircraft experienced a mechanical problem and was not brought down by hostile fire, but the cause of the crash was still under investigation.

The UH-60 Black Hawk was part of a pair of helicopters on a nighttime operation when the crash occurred. The four crew members and 10 passengers who perished in the crash were assigned to Task Force Lightning, the military said. It did not release identities pending notification of relatives.

The American military relies heavily on helicopters to avoid the threat of ambushes and roadside bombs – the deadliest weapon in the militants’ arsenal – and dozens have crashed in accidents or been shot down.

The deadliest crash occurred on Jan. 26, 2005, when a CH-53 Sea Stallion transport helicopter went down in a sandstorm in western Iraq, killing 31 American troops.

Today’s deaths raised to at least 3,721 members of the American military who have died since the Iraq war started in March 2003, according to an Associated Press count.

Meanwhile, a suicide truck bomber targeted a police agency in northern Iraq, killing at least 19 people and wounding 26, police and hospital officials said.

The attack occurred just before noon in Beiji, 155 miles north of Baghdad, and many of the casualties were civilians, according to the officials.

Iraqi police and soldiers have frequently been targeted by militants seeking to disrupt American-led efforts to enable the forces to take over their own security so foreign troops can go home.

A bomb and small-arms attack against a security post shared by police and American paratroopers also killed 13 Iraqi officers in Beiji in late June.

With violence unrelenting, political pressure mounted for Prime Minister al-Maliki to show progress in bringing Iraq’s battling factions together.

President Bush acknowledged his frustration with Iraqi leaders’ inability to bridge political divisions yesterday, but said only the Iraqi people can decide whether to sideline the troubled prime minister.

“Clearly, the Iraqi government’s got to do more,”Mr. Bush said at the close of a two-day North American summit with the leaders of Mexico and Canada.

The Sept. 15 deadline for Bush’s next progress report to Congress is fast approaching, leaving the president little time to show that his American troop buildup is succeeding in providing the enhanced security the Iraqi leaders need to forge a unified way forward.

American Ambassador Ryan Crocker, co-author of the highly anticipated report to Congress, said Tuesday that Washington’s blueprint for reconciliation was insufficient to win back control of Iraq. Congressional benchmarks such as laws to share oil revenue and reform security services don’t tell the whole story, he said yesterday.

Mr. Crocker and the American military commander, General David Petraeus, may be heading into a storm of discontent as they argue before Congress that American troops need more time in Iraq.

Last week, a stunning suicide bomb attack killed as many as 500 people in northern Iraq, an attack blamed on Al Qaeda in Iraq.

Mr. Crocker called Iraq’s problems difficult but fixable, arguing for more time for his diplomacy and operations by the bolstered American military force.

“Failure to meet any of them (congressionally mandated benchmarks) does not mean the definitive failure of the state or the society,” Mr. Crocker said. “Conversely, to make them all would not by any means mean that they’ve turned the corner and it’s a sun-dappled upland from here on in with peace and harmony and background music. It’s just a lot more complex than that.”

He echoed Mr. Bush’s frustration with the lack of action by Mr. al-Maliki government’s on key legislative measures.

“Progress on national level issues has been extremely disappointing and frustrating to all concerned – to us, to Iraqis, to the Iraqi leadership itself,” Mr. Crocker said. But he added that the Shiite prime minister was working “in the shadow of a huge national trauma.”

While saying American support was not a “blank check,” Mr. Crocker said Washington would continue backing Mr. al-Maliki’s government “as it makes serious efforts to achieve national reconciliation and deliver effective governance to the people of Iraq.” He stressed that it’s not just al-Maliki, but “the whole government that has to perform here.”

Mr. Crocker acknowledged “a lot of violence” in southern Iraq, where bombers killed Muthana province Governor Ali al-Hassani on Monday and Governor Khalil Jalil Hamza in neighboring Qadasiyah province nine days earlier.

Both governors were members the Shiite political powerhouse, the Supreme Islamic Iraqi Council, led by Abdul-Aziz al-Hakim. His loyalists dominate police in Iraq’s south and are fighting Mahdi Army militiamen for dominance in the region, which may hold 70 percent or more of Iraq’s oil reserves, according to various estimates.