ATHENS -- After its recent setback over maintaining U.S. F16s in Spain, Washington is bracing for a tough new round of base negotiations here this week with Greece.

At stake is whether the United States will be allowed to keep its four military bases after the end of the year. Spanish Prime Minister Felipe Gonzales has told Washington it must withdraw the 72 F16 fighter-bombers at Torrejon, near Madrid.

U.S. officials remain optimistic about their chances in Greece despite Prime Minister Andreas Papandreou's past threats to close the U.S. bases.

A six-man team of U.S. negotiators led by Ambassador Alan Flanigan is scheduled to meet Monday with its Greek counterparts here to negotiate a new agreement to replace the five-year base accord that expires Dec. 31.

U.S. optimism hinges on the perception that Papandreou, a Harvard-trained economist and Socialist who was a U.S. citizen for almost two decades before returning to Greek politics, has moderated his line during the seven years he has been in power.

Papandreou led his Panhellenic Socialist Movement to victory in 1981 on a platform that pledged to remove the bases and pull Greece out of the North Atlantic Treaty Organization and the European Community.

Since then, he has made his peace with the EC and is scheduled to take over the organization's rotating presidency this summer. He has also dropped all discussion of pulling Greece out of NATO. Earlier this month, he met Turkish Prime Minister Turgut Ozal in Switzerland to try to improve the strained relations that last year almost brought the two neighboring NATO allies to war.

Papandreou threatened to shut down the U.S. bases last summer because the State Department had accused Greece of negotiating secretly with Palestinian terrorist leader Abu Nidal to ensure a tranquil tourist season. Since then, the prime minister has toned down greatly the anti-American rhetoric that has served him so well in domestic politics.

"I think that the prime minister has decided -- for reasons that are not entirely clear -- that it would be wise to better relations with the United States," said one western diplomat.

It is expected that Papandreou will invoke the clause in the current agreement that allows either side to give five months notice to terminate the agreement.

He will do this, U.S. officials believe, to make the point to the more anti-American wing of his supporters that he is keeping his promise to terminate the agreement.

If no new accord is reached by year's end, the current agreement is terminated and the United States has 17 months to dismantle the military, communications, logistics and intelligence facilities.

Papandreou has said that any new agreement must first be approved in a referendum. National elections are also expected to take place before June, and Greek and western analysts here said he could try to link the elections to the referendum, further complicating quick approval.