WASHINGTON, July 16: The United States plans to develop an experimental 13,600-kg bomb, the biggest in its inventory, aimed at destroying deeply buried targets beyond the reach of existing bombs, the Air Force said on Friday.

The Massive Ordnance Penetrator, or MOP, would be about one-third heavier than the 9,500-kg Massive Ordnance Air Blast, MOAB, dropped twice last year in "live" tests at a range in Florida.

The Air Force's Air Armaments Center at Eglin Air Force Base, Florida, said it planned to award a contract for the "technology demonstration" work as early as mid-September, based on a perceived need for such a weapon.

"We think a bomb like this could be important in the future for targets that we can't destroy with what we now have," said Jake Swinson of the armaments center, which develops, tests, evaluates and acquires non-nuclear, air-dropped munitions.

Among companies that have built ground-penetrating bombs are three of the top US defence contractors, Lockheed Martin Corp., Boeing Co. and Raytheon Co.. Proposals are due by Aug. 16, the armaments center said.

The plan to develop the bomb was reported first by Jane's Defence Weekly. The project's demonstration phase would cost about $11.5 million through the end of fiscal 2007, it said.

Flight testing was projected in about 2006, Swinson said. Penetrators are made of special alloys designed to stay intact on impact to improve the effectiveness of conventional weapons against deep tunnels and other underground facilities.

The 21,000-pound MOAB, by contrast, is an experimental blast fragmentation bomb. Packed with 18,000 pounds of high explosive, it would be used against surface targets and explode at or just above the ground.

Swinson said the mammoth bomb might be designed to fit in the bomb bay of Northrop Grumman Corp.'s batwing B-2 radar-evading bomber as well as the Boeing B-52 bomber. The MOAB, a modernization of the Vietnam-era BLU-82 "Daisy Cutter" used to clear jungle helicopter landing zones, may be dropped only from slow-flying C-130 transport aircraft.

The new bomb would not be the largest ever built by the United States. A 44,000-pounder was carried by Cold War era B-36 intercontinental bombers but was never used. -Reuters