GENEVA (AP) _ Pakistani and Afghan negotiatiors failed Thursday to break a deadlock over the pullout of some 115,000 Soviet troops from Afghanistan, and the U.N.-sponsored talks seeking a political end to the six-year-old Afghan war were suspended.

United Nations mediator Diego Cordovez said four days of indirect talks between the delegations could not resolve an impasse over the format of negotiations on this ''key, central and critical issue.''

He said he submitted new proposals to break the deadlock for the two governments to consider until the talks resume in late February or early March. Cordovez said he may visit the Pakistani and Afghan capitals before then.

The two neighbors have conducted indirect talks for 31/2 years, with Cordovez shuttling between the delegations seated in separate rooms in the Geneva U.N. office, the Palais des Nations.

During the previous round of talks in August, Afghanistan demanded that the troop withdrawal problem be dealt with in face-to-face negotiations.

Pakistan does not recognize the Afghan government of Babrak Karmal and it rejected the demand, saying direct negotiations would legitimize what it considers a Soviet puppet regime.

Three other elements of a proposed settlement have been nearly completed since then: bilateral agreements on non-interference and on the return of about 5 million Afghan refugees from Pakistan and Iran, and draft declarations of international guarantees.

Pakistan has said it would consider direct negotiations only after all four elements of the settlement have been completed. Both sides reiterated their positions regarding the format in the sixth round, which began Monday.

Cordovez told a news conference: ''The question of the format of these talks (on the troop withdrawal) was again raised and it remains unresolved ... despite the sustained efforts made in a very constructive spirit.''

He said both sides had ''very strong feelings'' about this point.

Soviet troops intervened in Afghanistan in December 1979. They ousted one pro-Moscow regime and installed hardliner Karmal.

There had been hope for progress on the troop withdrawal issue following last month's summit between President Reagan and Soviet leader Mikhail Gorbachev.

Secretary of State George Shultz said after the summit that there were hints of greater Soviet flexibility. But Gorbachev has since reiterated that any withdrawal must be preceded by an end to Western aid to the anti-Marxist guerrillas.

Meanwhile, the official Radio Kabal reported from the Afghan capital that a deserter from Afghanistan's Border Security Forces was sentenced to death Thursday for joining the guerrillas fighting the communist government.

The broadcast, monitored in Islamabad, Pakistan, said Abdull Qahar was given the death penalty for murder, looting, armed revolt and working for the guerrillas. Qahar was a non-commissioned officer.

It was the sixth death sentence against a guerrilla announced by Radio Kabul in the past two weeks.

The broadcast also said new governors had been appointed in three provinces where the government has reportedly suffered setbacks in its war against the Moslem rebels.

It said governors were appointed for Helmand, Loghar and Bamyan provinces at a meeting Thursday of the Politburo of the ruling Afghan Communist Party. It did not say what had happened to the previous governors or why they had been replaced.