Soviet officials have threatened to cut off food, energy and raw materials to the three separatist Baltic republics unless they agree to remain in the Soviet central planning system and turn over their hard currency earnings to Moscow, Lithuanian President Vytautas Landsbergis said here yesterday.

Landsbergis, speaking at a news conference near the end of a two-day visit to Washington, said the ultimatum was delivered by Soviet Prime Minister Nikolai Ryzhkov and Planning Minister Yuri Maslyukov at a meeting with the Lithuanian, Latvian and Estonian prime ministers last month.

"The specific demand made on us," the Lithuanian president said, "was that we remain in the central planning, finance and taxation system and that we give over to the Soviet authorities the hard currency that we earn." Landsbergis said the Lithuanians were told they would be cut off from all Soviet shipments on Jan. 1, but he expressed confidence that his breakaway republic would survive.

"I am convinced that this winter is going to be a decisive one in our struggle," he said through an interpreter. "All forces and all measures possible are going to be used to destabilize the situation in Lithuania. . . . They hope that General Winter is going to be on their side, but we are convinced that we are going to fend off the attack and emerge victorious."

Landsbergis said his country would use tactics it developed last winter, when the Soviets first cut off shipments in retaliation for Lithuania's Feb. 11 declaration of independence. "We are signing agreements directly with Soviet republics, or our factories and enterprises are signing direct agreements with factories and enterprises in the Soviet republics," he said. "In fact, you are seeing economic perestroika {restructuring} in action."

Lithuanians already are storing up food against a cutoff, Landsbergis said. He conceded that there would be shortages of gasoline and that many Lithuanian workers could be unemployed unless supplies of metal and wood for factories can be replaced.

The Jan. 1 cutoff threat apparently has not been made against Latvia or Estonia, although their officials also rejected Ryzhkov's demands at the Nov. 10 meeting in Moscow. Nevertheless, Latvian leaders are reportedly braced for some form of crackdown after the Soviet Congress of People's Deputies, the nation's highest legislative body, convenes next week, and they have sent some officials abroad to escape possible arrest.

Landsbergis, who met Monday with President Bush, rejected suggestions that Washington had given Moscow assurances it would ignore a crackdown on the Baltics in exchange for Soviet cooperation on possible use of force against Iraq in the Persian Gulf crisis. "Especially after my meeting with President Bush," Landsbergis said, "I have no reason to think that way."