With negotiations for a Soviet troop withdrawal from Afghanistan at a crucial phase, internal developments in Afghanistan and Pakistan -- plus Soviet and U.S. interests -- are clouding prospects for further progress, according to diplomatic sources here.

As they have for years, Pakistani and Afghan diplomats will go to Geneva this week to sit in separate rooms of a U.N. building, while Undersecretary General Diego Cordovez shuttles between them to mediate.

But, unlike previous sessions, this month's round of what are called "proximity" talks will focus exclusively on the difficult central issues: How quickly would the Soviet forces leave Afghanistan, and at what point would Pakistan and the United States respond by cutting off support to the anticommunist Afghan resistance movement there?

Cordovez, who last year shepherded the two sides to agreement on several subsidiary issues, said last month, "We have entered a decisive stage."

"We will know in May," he said, "if there is going to be a settlement or not."

Pakistani analysts and western diplomats remain largely skeptical over chances for an agreement and suggest that internal events -- such as the departure from office of Afghan party leader Babrak Karmal and the return to Pakistan of opposition leader Benazir Bhutto -- bring new uncertainties to the talks. These observers also question how badly the United States and the Soviet Union want a political settlement, and suggest that the talks could drag on for a long time.

In March, Pakistan rejected an Afghan-proposed timetable for withdrawal, reported as one to two years long. Pakistani officials have suggested a pullout should be completed within six months or less.

In this week's talks, the two sides will consider a draft agreement, including a timetable, prepared by Cordovez but not made public.

The Soviet Union invaded Afghanistan at the end of 1979 to prop up a collapsing communist government, and installed Babrak as president. Moslem resistance fighters, called mujaheddin, based in Pakistan, receive covert U.S. arms supplies.

Afghanistan and Pakistan -- and their superpower backers -- say officially that they want a political resolution to the war, but neither side has convinced the other of its sincerity. "Each side believes that the other doesn't want a settlement," Cordovez said last month.

Last summer, the sides agreed to three "instruments" of a political settlement, providing that Afghanistan would take back the 2 million to 3 million refugees now in Pakistan, that the United States and Pakistan would stop their aid to the mujaheddin, and that the United States and the Soviet Union would guarantee implementation of any overall accord.

But in December, the Pakistani and Afghan negotiators deadlocked over the crucial "fourth instrument," a timetable for Soviet withdrawal -- and the linkage of such a pullout to a cutoff of aid to the mujaheddin. Afghan negotiators also demanded that the Pakistanis meet them face-to-face, an act of implicit recognition for the Kabul regime that Islamabad has always refused.

Last month, Cordovez announced that he had negotiated an "understanding" between the two sides on the format of the talks. A Pakistani political analyst familiar with the negotiations said last week that Kabul and Islamabad had agreed to keep talks on an indirect basis, but that the Pakistanis will meet the Afghans directly to sign any agreement and for subsequent negotiations on implementing it.

The Pakistani analyst, who asked not to be named, said Pakistan had rejected as "totally ridiculous" an Afghan timetable transmitted by Cordovez in March. "Our starting point in setting a schedule for Soviet withdrawal is the time that is logistically necessary," he said, noting that Pakistani officials have in the past suggested four months as an appropriate period.

In addition to the change of leadership in Afghanistan, the uncertainties of Pakistani politics also cloud the future of the talks. Ayaz Amir, a prominent political columnist, says popular opposition to the government's Afghan policy is underestimated.

"There is a growing awareness of the dangers to Pakistan of the millions of refugees and of supporting the mujaheddin," Ayaz said. Visitors to Pakistan's Northwest Frontier Province, where the concentration of refugees is greatest, hear the complaints of local residents about refugees holding jobs, owning land and grazing their livestock on the sparse forage.

The Pakistani left, including many supporters of Benazir Bhutto, is most vociferous in calling for recognition of the Kabul government. Since her return from exile, Bhutto has accused President Mohammed Zia ul-Haq of exaggerating the Soviet-Afghan threat to Pakistan, but she has not proposed a notably more conciliatory posture.

Some observers predict that, if Bhutto succeeds in her campaign to force a new election, she would win it and then face pressure from her backers to soften Pakistan's stance toward Afghanistan.

"If I were the Soviets or Afghans, I would be waiting to see if that might happen, rather than negotiating seriously right now," one western diplomat here said.

Diplomats here and Afghan mujaheddin officials in Peshawar, near the Afghan border, are skeptical that the Soviets really want to withdraw. "Gorbachev talks about peace, but all we see in Afghanistan is escalation," said Mohammed es'Haq, a spokesman for the Afghan Jamiat-i-Islami party.