U.S. officials said yesterday that they do not have a clear idea of why Greek Prime Minister Andreas Papandreou unexpectedly withdrew his support for the reelection of President Constantine Karamanlis or what effects the move is likely to have on the strained U.S.-Greece relationship.

Meanwhile, Defense Secretary Caspar W. Weinberger called the dumping of Karamanlis a "disappointment" and said it provides "further evidence of the very volatile nature and shifting positions" of the Greek leadership.

"Our understanding was that there had been a full promise to Mr. Karamanlis that had been rudely and abruptly halted and reversed," Weinberger told a group of visiting journalists from NATO countries.

Papandreou's weekend manuever forced the resignation of Karamanlis, a right-of-center former prime minister who is strongly pro-American. The personal prestige and influence he wielded in the president's office was regarded by many Greeks and by Greece's allies in the North Atlantic Treaty Organization as a force for keeping Greece tied to the West despite Papandreou's anti-American rhetoric and gestures toward neutralism.

On Saturday Papandreou withdrew a pledge that deputies of his Socialist Party would support Karamanlis' reelection bid when the parliament begins voting for president Friday. That means that Papandreou must seek an alliance with the Communist Party to muster the votes necessary to elect his new presidential candidate, Christos Sartzetakis, a supreme court judge.

What happens inside Greece "has potentially major implications for Greece's foreign policy and its relations with the United States," a senior State Department official said. "Having Karamanlis in the presidency gave Papandreou a sort of safety net with many voters who felt that Karamanlis would keep the country from swinging too far to the left."

However, the official continued, it is too early to tell whether Papandreou feels that he can improve his political position by adopting more radical positions. In the foreign policy area, that would mean increased criticism of the United States, possible further loosening of Greece's ties to NATO and additional attempts to cultivate relations with the Soviet Union and such radical figures as Libyan leader Muammar Qaddafi.

Papandreou's move was especially puzzling to U.S. officials because it came when he seemed to be trying to improve relations with Washington. In the face of increased U.S. disposition to respond sharply to his criticisms, Papandreou in recent weeks had toned down his rhetoric and taken a more conciliatory stance toward the United States.

The break with Karamanlis came on the heels of Papandreou's announcement that Greece will buy 40 U.S. F16 jets and 40 French Mirages. He also has curbed his rhetoric about terminating U.S. air and naval base rights -- threats that had caused the administration to begin contingency planning to move the bases in 1988.

For these reasons, some U.S. officials said, the latest political upheaval in Greece might prove to be nothing more than a tactical move by Papandreou to confuse his opposition. In the past, he has been successful in projecting an image of radicalism while cooperating quietly with the United States and NATO on such substantive matters as renewing base agreements