Two U.S. Marine helicopters with eight men aboard were reported missing in the Gulf of Oman Monday, and an RF-4C U.S. Air National Guard reconnaissance plane from Alabama crashed in Saudi Arabia Monday, killing both crewmen aboard.

Meanwhile, warships upholding the trade embargo forced two Iraqi ships to stop and submit to searches.The RF-4C crashed in southern Saudi Arabia about 1:10 p.m. local time, the Pentagon said. The identities of the two crewmen were withheld pending notification of next of kin.

The cause of the accident was not immediately known.

British, Australian and American warships fired warning shots across the bow of one ship to halt it in the Gulf of Oman. The freighter was empty and was allowed to continue, presumably to Iraq, British defense officials said.

No shots were fired at the second Iraqi ship in the Gulf of Oman, and it was still being searched late Monday, they said.

The two Marine helicopters disappeared about dawn, said Cmdr. J.D. van Sickle, a Navy spokesman.

He said the Navy had no idea what had happened to the UH-1 craft.

Each helicopter carried two pilots and two crew members. Their identities were withheld pending completion of the Navy's search in the Gulf of Oman, which is east of the Persian Gulf.

The helicopters were operating from the carrier USS Okinawa, one of more than a dozen ships deployed in the region after Iraq invaded Kuwait on Aug. 2.

The United States leads a multinational force of ships in the region and troops in Saudi Arabia, and the United Nations has imposed a land, sea and air embargo to isolate Iraq and try to force it to withdraw from Kuwait.

The warning shots fired at the Iraqi freighter marked the first time a British warship has shot to enforce the embargo and indicated a new teamwork is developing in the naval blockade's operation.

Three frigates - the HMS Battleaxe, USS Reasoner and AHMS Adelaide - fired at the Al-Wasitti in the Gulf of Oman, the British Defense Ministry said.

Royal Marines then boarded the 5,885-ton vessel, with a Coast Guard team from the Reasoner following them, the ministry said in London.

The second Iraqi ship, the Tadmur, was stopped off the Omani coast and was being searched by Royal Marines from the frigate HMS Brazen, the ministry said.

Meanwhile, a Kuwaiti official said his nation's government-in-exile will ask the United Nations to allow an airlift of medicine into Kuwait City to help people who are critically ill. "We are very concerned about people who are dying because of a lack of medical supplies," Suleman Mutawaa, the government's planning minister, told reporters in London.