Baghdad ? Fourteen U.S. soldiers were killed Wednesday when a Black Hawk helicopter crashed during a nighttime mission in northern Iraq, but the military said it appeared the aircraft was lost by mechanical problems and not from hostile fire.

It was the Pentagon’s worst single-day death toll in Iraq since January and indicated how forces are relying heavily on air power in offensives across northern regions after rooting out many militant strongholds in Baghdad and central regions.

But extremists are striking back. A suicide truck bombing against a police station in the northern oil hub of Beiji claimed at least 45 lives – 25 policemen and 20 civilians – amid a series of deadly attacks north of the capital.

The growing bloodshed in the north carries a mixed message. It suggests some success for the U.S.-led security sweeps seeking to reclaim control of areas in and around Baghdad. But it also highlights the apparent resilience of groups such as al-Qaida in Iraq as they retaliate and seek new footholds.

The UH-60 helicopter went down before dawn in the Tamim province that surrounds Kirkuk, an oil-rich city 180 miles north of Baghdad, said Lt. Col. Michael Donnelly, a military spokesman in northern Iraq.

He declined to be more specific about the location of the crash, but said the facts gathered indicated it was almost certainly due to a mechanical problem and not hostile fire. The final cause remained under investigation, however.

The Black Hawk was one of two helicopters and had just picked up troops after a mission when it crashed, Donnelly said. The four crew members and 10 passengers aboard were assigned to Task Force Lightning, but the military did not release further information about their identities pending notification of relatives.

In Washington, a defense official said the helicopter was from the 25th Infantry Division’s combat aviation brigade, based in Schofield Barracks, Hawaii.

A U.S. soldier also was killed and three others were wounded Wednesday during fighting west of Baghdad, the military said separately.

The total of 15 was the largest single-day death count since 25 U.S. soldiers were killed around the country Jan. 20, including 12 who died in a helicopter crash. The deadliest crash occurred Jan. 26, 2005, when a CH-53 Sea Stallion transport helicopter went down in a sandstorm in western Iraq, killing 31 U.S. troops.

Wednesday’s deaths raised to at least 3,722 members of the U.S. military who have died since the Iraq war started in March 2003.