Terrorists unable to attack U.S. diplomats and military personnel on the job overseas are resorting to attacks on Americans in off-duty hours, according to the State Department's chief of diplomatic security, Robert E. Lamb.

A pattern of such attacks, first noted in mid-1985, is causing serious concern within the administration and prompting discussion of new countermeasures, Lamb said in an interview.

"Our embassies have been well protected, and terrorists seem frustrated in their efforts to get past our security," said Lamb. As a result terrorists have begun to go after Americans away from their well-guarded offices, in moments of leisure or travel.

Among the examples of this were the shooting of four U.S. Marines at an outdoor cafe in San Salvador last June, the bombing of the crowded "La Belle" discotheque in West Berlin April 5 in which a U.S. serviceman was killed, and the shooting of a U.S. Embassy communicator on a street in Khartoum, Sudan, April 15.

"This will require additional security measures" to protect Americans during travel time and leisure hours, and will change the way American officials and their families live overseas, said Lamb.

Less casual contact with the foreign community, fewer public appearances, more planning and care about all aspects of life abroad are among the likely responses to the growing terrorist threats, said Lamb. Physical measures under consideration include more armored cars and lightly-armored vehicles for U.S. personnel and increased security for the residences of Americans abroad.

The "greatest payoff" in improved security in the new circumstances may come from more and better training for U.S. officials abroad and their families. More time spent in training courses at the Foreign Service Institute is in the cards for State Department officials and their adult family members, said Lamb. Mobile training teams abroad are likely to step their efforts to improve security consciousness after Americans leave the United States, he added.

The danger -- and therefore the countermeasures -- extend to Americans who were not considered major terrorist targets in the past, Lamb said. "It used to be that we had to protect ambassadors and deputy chiefs of mission. Now we're having to protect everybody," including the lowest ranking members of an overseas embassy.

"The situation is very dangerous today. The number of threats has been increasing, not just since last week's raid on Libya but for a number of months," according to Lamb.

In addition to the long-term upward trend of threats against Americans abroad, dramatic incidents such as the U.S.-Libyan clash in the Gulf of Sidra last month and the U.S. raid on Libya last week cause sharp temporary increases in threats. Last week about 12 credible bomb threats daily, on the average, were reported by U.S. diplomatic missions worldwide. This is about twice as many as were received in earlier months.

"Our embassies continue to be the primary target of threats, because they are the prime symbols of America," said Lamb. As the threats move outside to "softer targets" which are more accessable to terrorists, "Americans are going to be put under greaters stress." While emphasizing tighter security, he said, "we can't allow terrorists to isolate us" because that would negate the reasons for keeping diplomats, their families and other official personnel aboard.

Embassies and their personnel have long been threatened by a wide variety of terrorist groups with different causes and modes of operation, Lamb said. He described Libyan backing for existing groups as adding to the threat, saying that other groups have been "stimulated" by Libyan financing and encouragement.

Libyan-backed terrorism, he said, tends to be "more lethal" and targeted more precisely against Americans than most terrorist activity by other groups in the past.

At U.S. embassies abroad, the responses to terrorism are being coordinated by "emergency action committees" usually headed by the U.S. ambassador and involving key staff officers and security personnel, Lamb said.

In the longer run, a $4 billion program of rebuilding and improving embassy buildings will provide important help, he said. This program is now before Congress