GENEVA, MARCH 2 -- The chief U.N. negotiator of an almost completed peace accord ending the long Afghan war appealed to Pakistan today to drop its demand for the formation of a new coalition government in Kabul, a stand that is threatening to hold up the withdrawal of 115,000 Soviet troops from Afghanistan.

U.N. Undersecretary General Diego Cordovez, opening what is expected to be the final round of indirect talks here between Pakistan and Afghanistan, said the issue of a new government "should be left to the Afghans alone" to settle and not become the "subject of an international agreement."

"We would be establishing, in my view, an extremely unfortunate precedent," Cordovez said at a press conference here.

Pakistani President Mohammed Zia ul-Haq has said he will not sign the agreement with the present communist-dominated Kabul government and has insisted on the formation of another one acceptable to the U.S.-armed Afghan resistance before he does.

The issue of establishing a new government is not officially a subject of the indirect negotiations -- known as "proximity talks" -- being conducted here under U.N auspices between Pakistan and Afghanistan. Cordovez said it should "absolutely not" be added now as a new issue.

"Please think of the possibilities that would be opened if we were to decide in this context that the setting up of a government is going to be the subject of a negotiation and agreement at the international level," he added.

The U.N. mediator said the Afghan talks here, which began in June 1982, had reached "a very sensitive" point and had been well-prepared in advance to assure their success. He said he felt there was "a real determination" by all parties now to get a settlement "as soon as possible."

Because Pakistan has refused to recognize the Kabul government, Cordovez has to shuttle between two rooms on the first floor of the Palais des Nations.

Cordovez indicated that he was encouraged by the statement of Pakistani Minister of State for Foreign Affairs Zain Noorani, who did not immediately repeat Zia's demand and said Pakistan's official position would be decided after a meeting of all Pakistani political parties on Saturday.

Noorani said he would probably go back to Pakistan for that meeting. Observers here said they doubted there would be much serious negotiating until his return early next week.

But the Pakistani official said he would raise the issue at the talks here now, saying that "without such a government set up simultaneously with the withdrawal there is no earthly possibility of the refugees -- 3 million of whom are on our soil -- returning to their homeland."

Pakistan is under heavy pressure from the Soviet Union -- and some from the United States as well -- to drop its demand for the establishment of a new Afghan government before the agreement is signed.

Soviet leader Mikhail Gorbachev offered Feb. 8 to start the withdrawal of Soviet troops on May 15 and to complete the pullout within 10 months, provided the U.N.-sponsored peace accords can be signed here by mid-March. He also insisted that U.S. military aid for the Afghan resistance be ended in return for the Soviet troop withdrawal.

Cordovez refused to predict whether the negotiations would be completed by then. He said he would stay here "as long as necessary" to complete the agreement, adding, "Perhaps it will be signed earlier than you think."

The only sections of the peace accords yet to be filled in concern the details of the Soviet withdrawal timetable. The Soviets are expected to present those details through the Afghan delegation here in the next few days.

Secretary of State George P. Shultz told Soviet Foreign Minister Eduard Shevardnadze during their Moscow meeting Feb. 21-22 that the United States will be looking closely at those details before deciding whether to cut off its substantial military aid to the Afghan resistance.

Shultz told reporters traveling with him on his Middle East peace shuttle that he had given Shevardnadze a general idea of what the United States wants to see in the withdrawal timetable.

The Reagan administration has been pressing the Soviets to complete their pullout by the end of this year and to "front-load" it -- that is, to withdraw a large portion of their 115,000 troops in the first few months.

The Soviets have told the Americans informally that they plan to pull out half of the total within 90 days.

Cordovez said the U.S. demand for "front-loading" had at first proven "very controversial" but was now accepted by all parties. But he said it still had to be translated into "very specific terms," adding, "It's not going to be easy."

Cordovez refused to provide much detail about the complicated, four-part agreement, which calls for the United States and the Soviet Union to serve jointly as "guarantors."

The first of the four so-called "instruments" of the accord is a bilateral agreement on noninterference in Afghanistan's affairs; the second is a declaration on international guarantees; the third covers the return of refugees, and the fourth sets out the interrelationship of the four parts as well as the withdrawal of Soviet troops.