The US military said it was investigating reports that an aircraft went down Wednesday in Iraq. Witnesses said a helicopter had gone down in a field in the Sheik Amir area northwest of Baghdad, sending smoke rising from the scene, in a Sunni-dominated area between the Taji air base 20 kilometers (12 miles) north of Baghdad and Garma, 30 kilometers (20 miles) to the west of the capital. "We are looking into initial reports of a possible aircraft down," US military spokeswoman Lt. Col. Josslyn Aberle said. The reports came five days after a US Army helicopter crashed in a hail of gunfire north of Baghdad, police and witnesses said - the fourth helicopter lost in Iraq in a two-week span. The US command said two crew members were killed in that crash. Meanwhile, according to an Associated Press analysis of casualty records released Wednesday, more US troops were killed in combat in Iraq over the past four months - at least 334 through Jan. 31 - than in any comparable stretch since the war began. Not since the bloody battle for Fallujah in 2004 has the death toll spiked so high.