Baghdad, Iraq ? A Marine helicopter made a hard landing Monday in a remote desert area of Anbar province, injuring 18 people, the third U.S. aircraft to go down in the insurgent stronghold in two weeks.

The military also announced that three American soldiers were killed in a roadside bombing north of the capital on Sunday, putting December on track to be one of the deadliest months of the war.

The latest casualties underscored a major danger for Americans in Iraq, where the military relies heavily on air travel to transport troops and ferry officials and journalists to remote locations and to avoid the dangers of roadside bombs planted by insurgents.

The CH-53E Super Stallion, the U.S. military’s largest helicopter, was conducting a routine passenger and cargo flight with 21 people on board when it went down about noon, the U.S. command said, adding that hostile fire did not appear to be the cause.

Nine of the 18 injured were treated and returned to duty, it said. The military did not give the exact location where the hard landing occurred, saying recovery efforts were under way.

On Dec. 3, a Sea Knight helicopter carrying 16 U.S. troops went down in a lake, killing four. On Nov. 27, a U.S. Air Force fighter jet crashed in a field, killing the pilot. Both took place in Anbar, a volatile Sunni-dominated province the size of North Carolina.

Sunday’s roadside bombing that killed the three soldiers took place while they were on a late-night patrol north of Baghdad, the military said. Two soldiers were wounded.

The attack raised to 46 the number of American troops who have died this month, an average of 4.6 a day. By comparison, an average of 3.4 were killed each day in October, the fourth-deadliest month of the Iraq war with 105 deaths.

At least 2,934 members of the U.S. military have died since the U.S.-led invasion in March 2003, according to an Associated Press count.

Last week, the bipartisan Iraq Study Group concluded the U.S. could be out of Iraq by early 2008 if it dramatically increased the number of troops advising Iraqi units and threatened to cut off aid to the Iraqi government unless it met certain milestones. Bush administration officials are weighing the panel’s recommendations as well as other options, including a short-term buildup of U.S. troops in Iraq.