GENEVA, Switzerland (AP) _ Vice President George Bush on Saturday cancelled plans to travel to West Germany to welcome the remaining 39 American hostages from the hijacked TWA jetliner after a snag developed in plans to release the cavtives in Beirut.

Bush said earlier in the day that President Reagan asked him to fly to Frankfurt to greet the Americans, who were taken prisoner when Moslem gunmen hijacked the Athens-Rome flight and forced it to Beirut. He said then he would cancel a trip to Paris, the last stop on an 11-day tour through seven West European nations, to comply with the request.

The vice president later planned to stay overnight in Geneva and decide his travel plans Sunday morning, said a spokesman for the U.S. Embassy here. He demanded anonymity.

Bush had said the freed captives were expected in Frankfurt from Damascus, Syria, late Saturday. But the release effort later collapsed when Nabih Berri, head of the Shiite Moslem militia guarding the hostages, demanded assurances that the United States would not retaliate once the Americans were freed.

Bush, who originally was scheduled for a 24-hour visit in Geneva, met with U.S. and Soviet arms negotiators at the arms talks here.

On Saturday, Bush also addressed an international conference on nuclear proliferation, and called for closer international cooperation to eliminate terrorism.

''It is absolutely essential if we are ever to overcome this modern version of piracy,'' Bush said.

The number of terrorist acts worldwide has risen from about 500 in 1983 to 700 last year and ''perhaps 1,000 in 1985 if present trends continue,'' Bush said.

He cautioned that, ''we must be prepared for other more dramatic acts of terrorism in the future,'' and said the ''ultimate step of terrorism'' was the possibility of nuclear terrorism.

Bush made his remarks on the final day of a three-day conference on nuclear proliferation sponsored by the Groupe de Bellerive, a private forum founded by Prince Sadruddin Aga Khan, a personal friend of the vice president's.