WEST BERLIN, JUNE 11 -- The West German government today pledged publicly for the first time that it will try an accused Lebanese terrorist in West Germany for murder and hijacking if it does not extradite him to the United States to face the same charges there.

The announcement by chief government spokesman Friedhelm Ost represented a significant toughening of Bonn's position on Mohammed Ali Hamadei, a Shiite Moslem. He is currently imprisoned in Frankfurt and wanted in the United States for allegedly participating in the June 1985 hijacking of a TWA airliner and the murder of U.S. Navy diver Robert D. Stethem of Waldorf, Md., who was one of the passengers.

West Germany previously had not publicly ruled out swapping Hamadei for two West German businessmen held by kidnapers in Beirut, or trying Hamadei on minor charges in hope of winning the two hostages' release. The kidnapers, believed to include one of Hamadei's brothers, have threatened to murder the West German captives if Hamadei is extradited.

Ost said Chancellor Helmut Kohl did not tell President Reagan at the Venice economic summit that West Germany had reached a final decision against extraditing Hamadei. Reagan said the same thing at a news conference this afternoon.

Yesterday, White House spokesman Marlin Fitzwater told The Washington Post that Kohl had rejected a personal appeal from Reagan on Tuesday to extradite Hamadei and that Kohl had given the "clear impression" that Hamadei would be tried in West Germany. But Ost said, "As Chancellor Kohl told President Reagan in Venice, Mohammed Hamadei either will be extradited to the United States or will face charges of hijacking and murder before a German court. The government has not yet decided between these alternatives."

Reagan said today that Kohl "has never said outright to me, no extradition." The president emphasized, "The only question that remains is, will Hamadei be tried for murder and hijacking in the United States, or will he be tried for murder and hijacking in Germany."

Bonn government sources expressed surprise at Fitzwater's statements and said Kohl had decided before going to Venice that he would tell Reagan that a final decision on Hamadei still was pending.

"We are very sure about what Kohl said," a government source in Bonn said by telephone.

"My understanding is that there was a breakdown of the internal communication process in the White House," the source said. "Either the president didn't catch what was intended by Kohl, or Reagan didn't describe it correctly to Fitzwater."

The West German government decided to rule out swapping Hamadei or trying him on the minor charges in part because of lack of progress in efforts to obtain the release of the two hostages, the source said.

The West German news agency DPA quoted "reliable sources" as saying the Bonn government has reached a tentative decision to try Hamadei in West Germany rather than extradite him. The news agency also said Hamadei could be released and expelled from West Germany before completing his sentence if he is convicted.

Under West German law, Hamadei would face a mandatory life sentence with possibility of release only after 15 years if he were convicted of murder. He could be pardoned, however.

West German and U.S. officials noted that West Germany might prefer to extradite Hamadei, even at the risk of endangering the two hostages in Beirut, to avoid facing a terror campaign during a trial.

{U.S. Attorney for the District of Columbia Joseph E. diGenova, who with Justice Department attorney Karen Morissette is expected to try the case in Washington if Hamadei is brought to the United States, is expected to go to West Germany within a few days in a last-ditch effort to persuade the West German government to extradite Hamadei, according to sources, Washington Post staff writer Nancy Lewis reported.}