BAGHDAD, Iraq (June 21) -- Insurgents gunned down four U.S. Marines west of Baghdad on Monday, and South Korea said it would go ahead with plans to send thousands more troops to Iraq despite a threat by Iraqi kidnappers to kill a South Korean seen pleading for his life on a videotape.

A U.S. Army soldier was killed Monday and seven others were wounded in a mortar attack in north-central Baghdad, the U.S. command said. The casualties indicated no let-up in attacks against Americans as the June 30 transfer of sovereignty draws near.

A videotape delivered to Associated Press Television News showed four Marines in uniform lying dead in what appeared to be a walled compound in Ramadi, an insurgent stronghold 60 miles west of Baghdad. One of the Americans was slumped in the corner of a wall.

The bodies had no flak vests - mandatory for U.S. troops in contested areas - and at least one was missing a boot. One fieldpack was left open next to a body as if the attackers had looted the dead before fleeing.

Brig. Gen. Mark Kimmitt, coalition deputy operations chief, confirmed the killings but gave few details. He said a U.S. quick reaction force found the bodies after the troops failed to report to their headquarters as required.