Military officials in Iraq say they are searching for two American crew members who are missing after a U.S. Army helicopter crash west of Baghdad.

Four of the six people on board the UH-60 Blackhawk helicopter were injured when it went down Tuesday in Al Anbar province, an area of high insurgent activity.

The aircraft was on what the Army calls an area familiarization flight at the time.

U.S. officials say the helicopter's crash does not appear to be the result of enemy action.

Another wave of violence swept through Baghdad Tuesday, with at least 20 people killed in multiple attacks.

Two bombs that exploded minutes apart at a market in the center of the capital of Shurja killed 10 people and wounded nearly 70 others.

U.S. and Iraqi authorities are adding about 10,000 troops to the military contingents already in Baghdad. The reinforcements (about 3,500 American soldiers and 6,000 Iraqis) are intended to reverse the increase in bloodshed in the city, much of which is blamed on sectarian militias.