MOSCOW (AP) _ The Soviet Union said Saturday its troops in Afghanistan will cease fire on New Year's Day and appealed to the United States to support the truce, but U.S.-backed rebels vowed to keep fighting.

The Foreign Ministry said ''all units of Soviet troops that still remain in Afghanistan'' will lay down their arms on Jan. 1 and the Soviets also are ready to stop supplying weapons to the Kabul government. Afghan President Najib on Friday committed his forces to the truce.

''The Soviet Union fully supports this peace move of the Afghan leadership,'' the Foreign Ministry said in a statement.

President Mikhail S. Gorbachev called for the truce in a Dec. 7 address to the United Nations, but the United States rejected it.

On Friday, Alexander Bessmertnykh, a first deputy Soviet foreign minister, met with U.S. Ambassador Jack Matlock to make a bid for Washington's support.

The official Tass news agency said Bessmertnykh also made the pitch to the ambassador from Pakistan, which with the help of the United States supplies arms to rebels fighting Afghanistan's Marxist government.

''Hope is expressed in the Soviet Union that the Afghan opposition and all countries ... first of all the United States and Pakistan, will choose peace and through concrete actions respond to the peaceful intentions of the Republic of Afghanistan and the Soviet Union,'' the Foreign Ministry said in Saturday's statement.

The statement, carried by Tass, did not make clear how long Soviet troops would hold their fire if the rebels don't stop shooting. Najib, however, said Moslem insurgents will be given four days to accept the truce. If they refuse, government troops will start shooting again, he said.

Afghanistan's first deputy defense minister, Lt. Gen. Mohammad Nabi Azimi, noted in a Tass interview Saturday that the rebels had not adhered to previous truces.

''If the opposition does not agree to the cease-fire once again, the army will deal powerful strikes at extremists in several areas of the country after Jan. 5,'' Azimi warned.

A rebel spokesman in Islamabad, Pakistan, dismissed the truce.

''We did not start our fight because he asked us to,'' said Masood Khalili. ''And we won't stop because he asked us to.''

Khalili belongs to the Jamiat-e-Islami guerrilla group whose leader, Burhanuddin Rabbani, met in December with Soviet Deputy Foreign Minister Yuli Vorontsov.

The U.S.-backed guerrillas have been fighting Kabul's troops for 10 years. In December 1979, Soviet troops marched into Afghanistan to bolster the Kabul government.

The Foreign Ministry said both sides should stop aiding their allies in the war.

''Addressing specifically the United States, the Soviet side again reaffirms its readiness on the basis of reciprocity to discontinue all supply of arms to the warring sides in Afghanistan from Jan. 1,'' the statement said.

Secretary of State George P. Shultz has rejected the proposal, noting that the Soviets snubbed prior U.S. offers for mutual cessation of outside aid.

He said the rebels were justified in trying to seize territory as the Soviets withdraw ''because the people of Afghanistan, broadly speaking, do not accept the fact that the puppet regime is entitled to hold certain areas of that country.''

A senior Western diplomat, explaining why the United States will not go along with the aid cutoff, said the Soviets poured in a huge amount of military supplies to Afghanistan in preparation for the New Year's peace moves.

On May 15, the Soviet Union began withdrawing its estimated 100,000 soldiers from Afghanistan under a U.N.-brokered agreement. Half of the Soviet troops have left the country and the remaining half are to pull out by Feb. 15.

But Soviet officials say they have suspended their troop withdrawal temporarily because of continuing U.S. and Pakistani aid to the rebels.