Air Force officials grounded all training flights in the Persian Gulf region for 12 hours and ordered a review of their "entire flying program" in response to a series of air crashes this week that have heightened safety concerns and prompted congressional calls for an investigation of military aviation operations.

Air Force squadrons assigned to the Arabian Peninsula were ordered to halt all training sorties from noon until midnight Saudi time Wednesday and were required to conduct "safety awareness meetings" and "review what they need to do to fly more safely," Pentagon spokesman Pete Williams said yesterday.

The "flying stand-down" halted dozens of training missions among the hundreds of fighter jets, bombers and other Air Force craft operating in Saudi Arabia and nearby areas, although patrol and reconnaissance flights continued. Since Monday, two Air Force planes and two Marine Corps helicopters have crashed, killing 12 people and increasing to 31 the number of accidental deaths associated with Operation Desert Shield.

In addition to the problems in the Middle East, the Air Force has grounded its 97 B-1B strategic bombers in the United States as a safety precaution after an engine on one bomber disintegrated during a training flight last week. The flight restrictions have been in effect since Friday, making it the longest B-1B fleet grounding since the planes became operational four years ago. Officials said yesterday that some bombers remain on alert and could be used in event of a crisis.

Air Force Lt. Gen. Charles Horner, chief of U.S. Air Force operations in the Persian Gulf area, has summoned "all his flying wing commanders" to meet and reexamine flight operations in the dangerous desert environment, officials said yesterday.

"The entire flying program will be examined, and steps will be reinforced to provide a continuation of flying training programs that maintain the highest levels of combat readiness consistent with safe operations," an Air Force spokeswoman said yesterday.

Rep. Frank McCloskey (D-Ind.) yesterday asked the House Armed Service Committee to investigate aviation safety procedures in the Persian Gulf saying, "I am concerned that the pilots and crews are not being properly prepared to operate in this environment."

While the Air Force action has been the most dramatic response to the spate of mishaps, the Army, Navy and Marine Corps also have begun additional reviews of safety procedures, according to a Pentagon spokesman.

The Army recently announced it has barred newly arrived pilots from low-level, night helicopter operations until they complete intensive training programs and has begun installing new safety equipment on all Army helicopters in the region.

Marine Corps officials disclosed yesterday that some Marine helicopters in the gulf area were grounded for 48 hours last month after three helicopter crashes. That stand-down did not include the brigade whose two Marine UH-1N Huey helicopters disappeared in the Gulf of Oman Monday, killing all eight crew members, officials said.

Fourteen people have died in 20 airplane and helicopter crashes near the Arabian Peninsula in the last two months. The U.S. military now has about 700 fixed-wing combat airplanes and 250 gunship helicopters operating in the area. There are an additional 300 airplanes and 650 helicopters being used for transport and other support operations in the region.

"I think it's too soon to say precisely that the accident rate is high, especially if you consider the fact that we've moved what amounts to a mid-sized city 8,000 miles, which quates to more than 4,000 flights," Williams said yesterday. "You have to add that in to the whole operation here and not consider just the flights that are taking place now that we've gotten everybody over there."

Included in the 31 accidental deaths associated with the Middle East operation are 13 officers and troops who died when an Air Force C-5 transport plane destined for Saudi Arabia crashed shortly after takeoff from Germany early in the mission.

Military safety records have been increasingly scrutinized in recent years in the all-volunteer, high-technology military. The Navy, which has suffered only one helicopter crash during this Persian Gulf operation, imposed a servicewide safety stand-down earlier this year after a series of aircraft and other accidents.

The number of deaths from air accidents in the gulf operation so far has been slightly below the average number of air crash fatalities for the military in recent years. Over the last nine years, the military has averaged 18 air crash fatalities per month, according to the General Accounting Office. In the two months since Operation Desert Shield began, 27 people -- or about 14 per month -- have died as a result of crashes, including the incident involving the transport plane in Germany.

Williams and other military officials, who say investigators have not identified a pattern common to all of the accidents, note that flying conditions in the desert are extremely dangerous and are intensified by the nearby presence of hostile forces and the constant threat of imminent combat. "Pilots have to concentrate harder," Williams said.

"They have to depend more on their instruments because there's no horizon frequently." Officials also note that there are few landmarks on the desert surface to assist pilots with navigation.

"Given the amount of flying that has to be done, given the extraordinary circumstances and given the higher-than-ordinary operating tempo in the area, I think our service people are doing very well," Williams said. "But any accident is cause for concern."