DHAHRAN, SAUDI ARABIA, SEPT. 10 -- U.S. intelligence services have picked up the first indications that pro-Iraqi terrorist groups have begun surveillance of potential American targets here and elsewhere in the Middle East, according to sources here.

"There is some evidence that they are checking things out," said one source.

The U.S. military began stringent new security measures in the Eastern Province of Saudi Arabia last week, requiring more rigorous identity checks of all individuals entering the busy air base in Dhahran and tightening access to the flight-line areas, where U.S. transport planes and fighter jets operate.

American military officials would not say whether the new security measures were the result of the new intelligence reports.

Memories of the devastating bombing of the U.S. Marine headquarters in Beirut in October 1983, in which 241 servicemen were killed, permeate the American military operations here. From desert posts to the main Saudi air bases and ports, officers and soldiers are taking extraordinary security measures.

"The memory of Beirut lives," said Maj. Mark Kiersey of the 82nd Airborne Division. "The last thing we want is another Beirut."

Sources here say U.S. intelligence services have picked up indications that pro-Iraqi Palestinian terrorist groups are "showing signs of life" and are "in movement" in the Middle East.

"It's probably only a matter of time {before they strike}, whether inside or outside of Saudi Arabia," said another source.

The U.S. military has exerted a major effort to enforce strict security measures at its base camps, barracks areas and military installations here. Commanders in the field are ordered to select staging areas based as much on security as on tactical battlefield requirements.

"It's precautionary," said Lt. Gen. Walter E. Boomer, chief of the Marine Corps forces here. "We believe the potential for a terrorist attack exists. . . . We just don't want to take any chances."

At the entrance to a desert base camp for the soldiers of the Army's 82nd Airborne Division, a five-ton truck blocks the main gate. The camp is also ringed by cement-filled barrels. At a port where tons of military equipment are being unloaded from ships and where thousands of troops are accommodated in warehouses awaiting desert assignments, snipers are perched on rooftops throughout the huge naval complex.

Military public affairs officers who escort media representatives onto military bases conduct bomb searches of their cars and buses every morning before they start the engines. Military troops are constantly reminded to report sightings of strangers or unusual activities near their facilities.

Military officials were so concerned about security in the capital of Riyadh that they ordered one hotel to evict all of its guests and allow only military operations in the hotel. A metal detector was erected at the door to screen all individuals entering the building. Even so, military authorities considered the site a potential terrorist target and have begun moving service officials to more secure military compounds.

U.S. military officials say the Saudi military, even before the current operation, had equipped its facilities with state-of-the-art security systems. Sophisticated cameras ring the desert perimeters of most Saudi airfields where the U.S. military is conducting major operations.

"We're doing everything we can to ensure the safety of our troops, but I could never say we're 100 percent protected," said Gen. H. Norman Schwarzkopf, commander in chief of all U.S. forces in the Middle East.

The most proficient Middle East terrorist organization is the Fatah Revolutionary Council led by Abu Nidal, who recently changed his base of operations from Tripoli, Libya, back to Baghdad, where he had his headquarters until the early 1980s.

Iraqi President Saddam Hussein, then seeking better relations with the United States, expelled Abu Nidal from Baghdad, a move that led to a Reagan administration decision to take Iraq off the list of states supporting terrorism. Following Iraq's occupation and annexation of Kuwait in early August, the Bush administration put that country back on the list of "terrorist nations."

A number of other pro-Iraqi Palestinian groups operate from Baghdad, including one led by Abul Abbas, whose operatives were responsible for the hijacking of the Italian cruise ship Achille Lauro in October 1985.

Security here in the kingdom has long been extremely tight because of the eight-year Iran-Iraq war during which Iran was constantly seeking to subvert Saudi Arabia because of its support for Iraq. U.S. facilities such as the embassy in Riyadh and the consulate here in Dhahran have taken extraordinary security precautions since the start of the Iran-Iraq war to guard against pro-Iranian terrorist attacks.

The embassy reportedly has received bomb threats since the arrival of American military forces here but had been the object of such messages before then. Nonetheless, they are being taken more seriously in the current climate of concern.

The main likely perpetrators of terrorist activities here are pro-Iraqi Palestinians and Yemenis, sources said. The Palestine Liberation Organization and Yemen were among the few Arab supporters of Saddam's action in seizing Kuwait.

At least 300,000 Palestinians and 500,000 Yemenis live here, compared to only 4,000 Iraqis. Yemenis never have been required to obtain visas to enter this country.

The Iraqis might also try to use the Kuwaiti passports seized from the foreign ministry when they occupied Kuwait City, although sources here say their numbers are known and Saudi border police are on the alert for Iraqis seeking to use forged Kuwaiti documents.