A bomb went off in a discotheque in West Berlin early this morning, killing at least two people and injuring up to 150 others, U.S. and Berlin officials said. The Defense Department reported at least 30 Americans were hospitalized.

The disco, called La Belle, is frequented by American GIs, Pentagon officials said. But they emphasized that they had only preliminary information on the bombing and could not be certain if the dead were military personnel, although they had been told that about 50 of the injured were Americans.

A statement released by U.S. military authorities in Berlin said "there appeared to be a number of fatalities." Military officials did not elaborate on their statement, and representatives of the U.S. Embassy could not be reached for comment.

A Berlin police spokesman said in a telephone interview that the two known dead were identified only as a woman and a man. Their nationalities were not known.

The bomb went off at 1:49 a.m. West German time (6:49 p.m. EST), officials said, tearing down walls and setting the building on fire. About 500 people were in the disco, the police spokesman said.

The disco club is located in Friedenau, a part of the Berlin-Schoeneberg District.

Berlin police said their preliminary investigation showed that the bomb contained 6 1/2 to 11 pounds of explosives. Defense Department officials here had no reports on what kind of bomb it was or who might have set it off.

In California, the White House said President Reagan, vacationing at his ranch near Santa Barbara, was notified of the bomb attack at about 8 p.m. PST (11 p.m. EST) by National Security Council deputy Donald Fortier.

"We have received reports that a number of people have been injured, including some Americans and apparently there have been some fatalities, but we have no details," White House spokesman Peter Roussel said.

The explosion occurred in a city where U.S. intelligence had reported threats by Libyan-directed or -influenced groups during the past week. Washington officials said the governments of East and West Germany had been alerted to the possibility of terrorist activities in Berlin.

The German city is among several in Europe where Libyan agents are said to have been active in recent weeks. If an investigation of the bombing suggests Libyan involvement -- as some U.S. sources expect -- the Reagan administration would be confronted with a clear-cut question of taking action against the source of the terrorism.

The bombing came shortly after American diplomats and diplomatic facilities in Western Europe and the Middle East were placed on an unusually high state of alert.

"Our people are on tiptoe and may have to be for months to come," a senior U.S. official said earlier yesterday about security precautions being taken abroad. He said foreign governments in Europe and the Middle East have been asked to cooperate by stepping up their intelligence and security around potential U.S. targets.

Such increased alertness has apparently paid off in France. According to a U.S. source, the French have expelled four persons -- an Algerian, a Tunisian and two Lebanese -- on suspicion of planning an attack on an American consular building in Paris.

Officials said that reports reaching Washington in recent weeks indicated that Libya was asking its allies and agents abroad to take action against U.S. targets in retaliation for what Libyan leader Muammar Qaddafi has charged is a U.S. campaign against him.

Concern within the administration focused in particular on reports of Libyan-sponsored surveillance of U.S. government and commercial facilities and officials. One such report, attributed to the Central Intelligence Agency in the current issue of Newsweek magazine, said that Qaddafi's agents have had "no fewer than 35" American targets under surveillance abroad, ranging from the offices of U.S. companies to the headquarters of the 6th Fleet and homes of its top officers in Naples, Italy.