DHAHRAN, SAUDI ARABIA, OCT. 30 -- In the most serious naval accident since Operation Desert Shield began, 10 American sailors on the USS Iwo Jima died today when a boiler room pipe ruptured, blasting them with high-pressure, super-heated steam.

In another accident, a Marine was killed and three injured when their vehicle overturned in the Saudi desert during a night exercise. Three other sailors were injured Monday when a Marine sentry inexplicably fired at their vehicle, a spokesman said.

The day's deaths raised to 42 the total of U.S. personnel killed since the military buildup against Iraq began. In addition, one serviceman has committed suicide, military officials said.

The accident on the Iwo Jima occurred as the amphibious assault ship, which had just completed a port call for routine maintenance in Manama, Bahrain, was about to join a 10-day U.S. Navy and Marine training exercise in the northern Arabian Sea, military officials said.

The beach assault exercise, called Sea Soldier II, is the second in the region since American forces were ordered to the Persian Gulf in early August after Iraq invaded Kuwait.

The accident occurred at 8:15 a.m., military officials said, when the steam, normally under 600 pounds per square inch of pressure and as hot as 850 degrees, spurted from a ruptured pipe, killing six crewmen outright. Four others, critically burned by the steam blast, died after being transferred to the hospital ship USNS Comfort.

The Iwo Jima "was only a couple of miles from the pier when the accident occurred," Lt. Kevin Wensing, a Navy spokesman, told the Associated Press.

Navy spokesman Cmdr. J.D. Van Sickle said the ship, which is based in Norfolk, Va., had 685 Navy personnel and 1,100 Marines aboard. It was towed back to port.

Other Navy officials said that such boiler room accidents occasionally occur, regardless of the vessel's operating area.

The Navy identified eight of the dead, but withheld two names until relatives could be notified. Those identified were David A. Gilliland, 21, Warrensburg, Mo.; Mark E. Hutchison, 27, Elkins, W.Va.; Daniel Lupatsky, 22, Centralia, Pa.; Daniel C. McKinsey, 21, Hanover, Pa.; Fred R. Parker Jr., 24, Reidsville, N.C.; James A. Smith Jr., 22, Somerville, Tenn.; John M. Snyder, 25, Milltown, N.J.; and Robert L. Volden, 28, Rego Park, N.Y., according to the Associated Press.

The Marine who was killed today, identified as 1st Lt. Michael N. Monroe, 27, of Auburn, Wash., died when his vehicle, one of the new wide-bodied jeeps now being used by the U.S. military, "went over an embankment and dropped approximately 20 feet," according to a military press release.

The accident occurred at 2 a.m., during a nighttime exercise in which the Marines were moving their desert camp from one site to another, a Marine spokesman said. Two of those injured were treated and released. The third was listed in good condition in a hospital.

The Marines were in a High-Mobility Multipurpose Vehicle, known as a "Humvee" from its initials, the spokesman said, but he could not say whether Monroe was the driver, whether the driver was using night-vision goggles, which have caused problems in the past, or exactly how the accident occurred.

A recent issue of Desert Dragon, a newsletter of the Army's 18th Airborne Corps, warned of the hazards of driving straight over desert sand dunes. Saudi Arabia's prevailing winds from the north cause dunes' southern faces to drop off sharply, sometimes 100 feet or more.

"Investigations show when a driver approaches the crest of a sand dune at a 90-degree angle, the hood of the vehicle" blocks the view of the precipitous drop on the southern side, the newsletter said.

Officials also reported today that three sailors traveling in a military pickup truck were injured Monday afternoon when a Marine sentry "apparently accidentally discharged" two .50 caliber machine-gun rounds at them. The sailors were "approaching the entrance to a facility" being used by Marines, military officials said.

One of the sailors was hit by bullets, one was injured by glass shards and one sustained a concussion. Authorities said the incident is under investigation.