The crash Sunday of an Air Force B-52 bomber in the Indian Ocean as a result of apparent mechanical problems does not signal that the venerable warplane has finally outlived its usefulness, military officials say.

While returning from a bombing mission, the B-52 went down en route to the Diego Garcia air base located on an Indian Ocean island 2,500 miles southeast of Saudi Arabia, Pentagon officials said.For more than three decades, the B-52, nicknamed the Stratofortress, has served as the primary strategic bomber for the United States. The massive warplane is a long-range heavy bomber capable of flying at speeds nearing the sound barrier and at altitudes up to 50,000 feet.

B-52s, used extensively for bombing in Southeast Asia during the Vietnam War, can carry conventional or nuclear weapons. In the Persian Gulf war, it has been used to pummel Iraqi troop concentrations and blast their fortifications, particularly in Kuwait. The B-52s and KC-135 tankers are the oldest active planes in the Air Force, with both rolling off the assembly lines in the late 1950s.

"The B-52s are quite capable," said Air Force Maj. Dick Cole, an air force spokesman and a former B-52 crew member. "We've pumped a lot of money into keeping it in a capable position. It's not the most glamorous plane, but it does its job. It's a workhorse."

Military officials say there is "no evidence" the crash resulted from hostile fire, saying the "best estimate" is that it was caused by a mechanical problem. Three crew members were rescued, while three were listed as missing.

Asked if the bomber was reaching the end of its useful life, Marine Maj. Gen. Robert Johnston said during a Sunday briefing in Riyadh, Saudi Arabia: "It would not appear so. Keep in mind that if you're trying to measure a B-52 in the Vietnam era vs. the capability we have now, it really is like apples and oranges. It's the same aircraft, but a much-enhanced targeting capability."

The first B-52s were built in 1954 and were designed exclusively to carry nuclear weapons. The B-52-G version being used during Operation Desert Storm was delivered by Boeing between 1957-59. The B-52s have been modified several times since then for modernization - including the addition of improved weapons delivery systems, anti-ship missiles and externally carried air-launched cruise missiles - at a total price tag of $5.5 billion, Cole said.

The B-52s used now measure 185 feet in wingspan, 40 feet in height and 160 feet in length and tote up to 70,000 pounds of ordinance. It cost $5.6 million per plane when delivered - "a bargain," said Cole.

This is the first B-52 lost in the gulf war; 29 were lost in Vietnam.