U.S. commanders in Iraq have asked for more of the rounds, said Lt. Col. Kevin Curry, an Army spokesman at the Pentagon, who added that early versions had already been used there in "limited numbers." More than 1,870 Americans have been killed in Iraq since the war began in March 2003.

The "thermobaric" Hellfire AGM-114N warhead creates an intense, sustained pressure wave that can strike around corners in "caves, bunkers and hardened multi-room complexes," the manufacturer, Lockheed Martin Corp., said.

The Bush administration and Lockheed Martin, the Pentagon's No. 1 supplier, have described the 27.5-pound (12.4-kg) warhead as highly lethal in enclosed structures while causing only minimal damage nearby.

"In the coming years, there are going to be some awfully surprised terrorists when the thermobaric Hellfire comes knocking," Bush said in a May 27 commencement address at the U.S. Naval Academy.

Lockheed is supplying the Army with 900 AGM-114N warheads under a $90 million pre-production contract awarded in December. The contract also included 180 AGM-114K missiles, the high-explosive anti-tank (HEAT) round and conversion of 100 HEAT rounds to the thermobaric configuration.

The AGM-114N is one of four variants of the Hellfire 2 family of missiles that has been used in Iraq, with more than 1,000 missiles fired to date, said Lockheed Martin's Orlando, Florida-based Missiles and Fire Control business unit.

Unlike conventional warheads, which unleash a sharp pressure spike that decays quickly, the thermobaric round includes a highly flammable, fluorinated aluminum powder to make its explosion push harder and last longer.

Designed, developed and built at Naval Air Systems Command in China Lake, California, the thermobaric round was put on a fast track to full production, said Jennifer Allen, a Lockheed spokeswoman.