Fourteen US soldiers were killed 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. Wednesday's crash 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. The UH-60 helicopter went down before dawn in the Tamim province that surrounds Kirkuk, an oil-rich city 290 kilometers 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.