US and Iraqi troops have killed 38 militants in fierce clashes in Baghdad, including 22 who attacked a military checkpoint in a Shiite militia stronghold, the US military said Monday. Suspected Shiite extremists, apparently taking advantage of a sandstorm that blanketed the capital, attacked several checkpoints and hammered the U.S.-protected Green Zone in the fiercest salvo in weeks on Sunday. The sandstorm had grounded the American aircraft that normally prowl for launching teams. On Monday morning, the insurgents lobbed more rockets or mortar shells toward the Green Zone, which houses the US embassy and much of the Iraqi government on the west side of the Tigris River. Alarms could be heard and the public address system in the area warned residents to take cover and stay away from windows.