VENICE, JUNE 10 -- West German Chancellor Helmut Kohl has rejected a personal appeal from President Reagan to extradite an accused Lebanese hijacker to face murder charges in the United States, White House spokesman Marlin Fitzwater said today.

Fitzwater said Reagan had pressed for the extradition of Mohammed Ali Hamadei during a private conversation last night with Kohl at a working dinner of leaders attending the economic summit here.

The White House spokesman said Kohl had agreed with Reagan that Hamadei should be tried for murder but turned down the extradition request. The West German leader left "the clear impression" that Hamadei would be tried on murder charges in West Germany, Fitzwater said.

Hamadei, a Lebanese Shiite Moslem, allegedly participated in the June 1985 hijacking of a TWA airliner to Beirut and the murder of U.S. Navy diver Robert D. Stethem of Waldorf, Md., who was one of the passengers.

State Department spokeswoman Phyllis Oakley, speaking before Fitzwater's remarks were available in Washington, told reporters there that the United States had been "assured that the German government will fulfill its responsibilities to extradite or try Hamadei" for seizure of the TWA flight and the murder of Stethem.

Later, after checking with U.S. delegation in Venice, Oakley said, "We stand by the {earlier} statement." She did not elaborate, but other State Department sources said the United States believes it has a commitment from the West Germans that if Hamadei is not extradited, he will stand trial in West Germany for murder and hijacking and not be tried on lesser charges, as unconfirmed reports from Lebanon had suggested last month.

The fate of Hamadei has been a sensitive one for the Kohl government because two West German businessmen are being held hostage in Lebanon by terrorists reputedly connected to Hamadei. They have threatened to kill the Germans if Hamadei is extradited.

Nonetheless, the Reagan administration had hoped West German authorities would accede to insistent U.S. demands for Hamadei's extradition. An administration legal team met with German authorities in Bonn earlier this week in the hope that progress toward an extradition agreement could be made in time for Reagan's visit to West Berlin and Bonn on Friday.

White House officials said the West German authorities had rejected the U.S. request, apparently out of concern for the safety of the West Germans held in Lebanon. Reagan then decided to approach Kohl directly, the officials said.

Fitzwater declined to characterize Reagan's reaction to Kohl's refusal of the extradition request other than to say, "We would have preferred to have him extradited."

But Reagan administration officials expressed relief that the Germans at least seem to have backed away from negotiating a rumored deal that U.S. officials said would have been a capitulation to international terrorism.

U.S. officials have said for the last month that they had assurances from the West German government that Bonn would try Hamadei for murder and air piracy in West Germany, and thus satisfy its legal obligations, if it did not extradite Hamadei to the United States. Bonn officials, without publicly confirming those U.S. accounts, have indicated that they were correct.

The Bonn government is extremely reluctant to comment publicly on the case, out of fear that to do so could endanger the two West German hostages. The only reports indicating that West Germany was proposing to try Hamadei on lesser charges came from unidentified sources close to the kidnapers in Lebanon. Those accounts were later disputed by West German and U.S. officials.

The West German government has been leaning since February toward trying Hamadei in West Germany, according to Bonn government sources. But the U.S. statement last month provided the first concrete indications that Bonn was determined to try him for murder and air piracy.

Under the proposed deal reported by sources close to the kidnapers in Lebanon, the West German businessmen would have been released in return for the trial of Hamadei and his brother, Abbas Ali Hamadei, also held in a West German prison, on lesser charges of possessing explosives and use of a false passport.

Such a deal could have resulted in the release of the Hamadeis after serving six-month sentences.

The U.S. legal team that sought Hamadei's extradition in Bonn this week included Abraham D. Sofaer, State Department legal adviser; Stephen S. Trott, associate attorney general; and Victoria Toensing, a deputy assistant attorney general from the Justice Department's Criminal Division.

Two hours before Fitzwater revealed that Reagan's request to Kohl had been turned down, White House chief of staff Howard H. Baker told ABC News in a taped interview that "the issue is not yet resolved" but had been "discussed at the highest levels."

Baker did express confidence that Hamadei "would be tried on a murder charge" in West Germany if he is not sent back to the United States.Washington Post staff writer John M. Goshko contributed to this report.