BAGHDAD — A soldier killed over the weekend south of Baghdad was the first American casualty in a roadside bomb attack on a newly introduced, heavily armored vehicle, a military spokeswoman said today.

The V-shaped hull of the huge MRAP — Mine-Resistant, Ambush-Protected —truck is designed to deflect blasts from roadside bombs, a weapon that has killed more American soldiers than any other tactic used by insurgents in Iraq.

The soldier who died Saturday was the gunner who sits atop the MRAP vehicle. Three crew members tucked inside the cabin were wounded. The vehicle rolled over after the blast, and it was not clear how the gunner died — from wounds in the explosion or in the subsequent roll-over.

There now are more than 1,500 of the costly vehicles in service in Iraq, and the Pentagon is working to get at least 12,000 more, using $21 billion provided by Congress. MRAPs cost between $500,000 and $1 million, depending on their size and how they are equipped.