The Soviets have begun preparations to remove the first of their 115,000 soldiers from Afghanistan despite an impasse in United Nations-sponsored negotiations in Geneva, according to administration sources.

"The intelligence community is unanimous in reporting that preparations for a withdrawal have commenced," said one administration analyst.In the past, the Soviets have removed some peripheral units, such as anti-aircraft batteries, but replaced them with others. The current preparations are of a different character, said the sources, but they declined to give specifics.

And they said they could only speculate about the exact time, duration and manner of a Soviet withdrawal.

In a parallel development, Afghan rebel leaders in Pakistan said over the weekend that the United States has stopped supplying them with Stinger anti-aircraft missiles.

Mohamad Nabi Mohamadi, leader of the Harekat Islami, one of the seven guerrilla groups fighting communist forces in Afghanistan, said the development comes in apparent U.S. anticipation of a settlement in the war.

Soviet Foreign Minister Eduard A. Shevardnadze has said the Red Army will withdraw from Afghanistan even if Afghan and Pakistani negotiators fail to reach agreement at United Nations talks in Geneva.

The talks are stalled over a demand introduced by the United States last December that the Soviets stop aiding the Kabul regime at the same time that Washington stops its covert aid program to the Mujahedeen.

Whether or not an agreement is reached, the Soviets are expected to start a withdrawal by pulling units of their 40th Army from garrisons south of Gardez and from Kandahar, Shinand and Herat in the West, according to administration officials.

These units are the most exposed to attack from a Mujahedeen force estimated by administration analysts to number about 200,000.

"Evidence is mixed on whether they (already) have removed some non-essential personnel" such as dependents and military units not directly involved in the fighting, said the administration analyst, who spoke on condition of anonymity.

If the U.N. talks produce an agreement, the Soviets are thought likely to leave elite special purpose forces to provide a security shield for the Kremlin-backed regime in Kabul, while moving the bulk of their departing forces to the Soviet Union, possibly to an area just north of the border.

If the U.N. talks collapse, and the Soviets decide on a unilateral withdrawal, "all bets are off," said one analyst.

One likely option, however, would be for the Soviets to concentrate forces around Kabul and within a recently created province in the far north, along the Soviet border.

The new province of Sari Pull was created from parts of Balkh and Juzjan provinces, which are separated from the Soviet republics of Uzbekistan and Tadzhikistan by the Amu Darya River. The Mujahedeen have staged a number of raids, none very destructive, across the remote border, according to Afghan and Soviet reports.

The largest town in the new province is Mazar-i-Sharif, which U.S. analysts have long considered a likely final staging area for a Soviet withdrawal.

Morale is already bad among the 40,000 Afghan government forces, according to the guerrillas, who say the confidence of the government has been shaken badly by the Soviet announcement that the Red Army will leave.