Cautiously optimistic that West Germany might now be more disposed to extradite a Lebanese man accused by the United States of murder and hijacking, a high-level U.S. delegation met with officials in Bonn yesterday to press the U.S. argument that the alleged terrorist should be sent here for trial.

U.S. officials said privately there is still no assurance that Bonn, concerned about the safety of two West German businessmen held hostage in Lebanon, will agree to extradite Mohammed Ali Hamadei. He is a Lebanese Shiite who allegedly took part in the June 1985 hijacking of a TWA airliner to Beirut and the murder of a U.S. Navy diver, Robert D. Stethem, of Waldorf, Md., who was among the passengers.

However, the sources continued, the Reagan administration now believes that heavy American pressure has deterred Chancellor Helmut Kohl's government from reaching an agreement with Mideast terrorists. That agreement would have seen Hamadei and his brother -- Abbas Ali Hamadei, also held in a West German prison -- tried in West Germany on lesser charges and given six-month sentences.

Representing the administration in yesterday's talks were Abraham D. Sofaer, the State Department legal adviser; Stephen S. Trott, the associate attorney general, and Victoria Toensing, a deputy assistant attorney general from the Justice Department's Criminal Division.

U.S. sources said that they were in Bonn to clarify West German questions about U.S. legal procedures and how they apply to Mohammed Hamadei. However, the sources said, the delegation's purpose also was to signal U.S. impatience over Bonn's failure to deal with Hamadei, who was arrested at the Frankfurt airport Jan. 13.

His arrest, and that of his brother a few days later, became a highly sensitive issue for the Kohl government when two Germans, Rudolf Cordes and Alfred Schmidt, were kidnaped by Beirut terrorists with reputed ties to the Hamadeis. In subsequent months, Bonn has followed a policy of assuring the United States it is weighing the extradition request, while floating trial balloons in the West German press about the possibility of a deal that would free the German hostages.

Two weeks ago, Hans-Juergen Wischnewski, an opposition Social Democrat who has extensive ties in the Middle East, flew to Tehran in an apparent attempt to open contact with the kidnapers. That led to reports of a deal under which the Hamadei brothers would be tried on minor charges and given sentences of about six months, with the German hostages being freed.

According to U.S. sources, that prompted the Reagan administration to mount a vigorous campaign to convince Bonn that such a move would be a major setback for efforts to combat international terrorism and might do serious damage to U.S.-West German relations.

The sources said the U.S. effort apparently succeeded in causing Bonn to back away from the deal. Now, under West German law, Bonn must decide soon whether to drop the lesser charges under which the Hamadeis are being held.

The U.S. view, which the sources said was pressed in yesterday's talks, was that these do not hold the promise of sufficient punishment and that if Bonn really is serious about fighting terrorism, it has no option other than to extradite Mohammed Hamadei to this country.