An American soldier was killed Tuesday when insurgents tossed a grenade at an Army Humvee (search) as it drove down a road Baghdad (search), the military said.

Another soldier was injured and taken to a military hospital where he was listed in serious condition. There was no word on Iraqi casualties.

Both soldiers were part of the 1st Armored Division (search). Their names were being withheld.

The military said it didn't believe the attack was related to the bombings at Shiite shrines in the capital in Karbala, which left several dead and injured.

The latest death brings to 548 the number of U.S. service members who have died since the United States launched the Iraq war in March. Most have died since President Bush declared an end to active combat May 1.