Afghan negotiators for the first time have mentioned a specific timetable for the withdrawal of Soviet troops in their talks with Pakistani officials in Geneva, according to sources close to the talks, but the proposal is being portrayed as completely unrealistic.

The future of the long-running Geneva talks is expected to be at the center of discussions in Washington this week between Pakistani Foreign Minister Yaqub Khan and U.S. officials, including Secretary of State George P. Shultz, according to diplomats from both countries.

While Pakistani and U.S. diplomats roundly reject a reported Afghan proposal of a four-year timetable for the pullout of Soviet forces from Afghanistan under a comprehensive settlement, there are indications of some emerging differences between them on the continuing guerrilla war.

U.S. officials in recent discussions labeled the Afghan-Soviet position as "ridiculous" and as a prescription for turning Afghanistan into another Soviet republic.

Pakistani officials are more circumspect, although they, too, are adamant that anything longer than a six-month withdrawal proposal is unrealistic.

These same officials, however, are quick to caution against sudden changes in the guerrilla alliance facing the estimated 120,000 Soviet troops inside Afghanistan. On the other hand, U.S. officials are believed to be anxious to see the alliance take on a more permanent structure.

Pointing to almost daily cross-border incidents, shellings and to the recent air clash in which two Afghan jets were shot down by U.S.-supplied F16s, the Pakistanis regularly strike a more cautious posture and emphasize areas of progress in the talks, under the sponsorship of U.N. Undersecretary General Diego Cordovez.

While there was general disappointment that the talks, suspended on May 23, failed to make significant progress on the withdrawal issue, officials close to the discussions say that all peripheral issues have been settled, bringing into sharp focus the two critical questions of withdrawal and monitoring of an overall agreement to end the bloody war.

Cordovez's strategy appears to be to tackle the monitoring issue first, apparently seeking agreement on a direct U.N. role or possibly a U.N.-sponsored monitoring force. Officials familiar with his assessment of the talks believe he is hopeful of progress on this issue at the resumption of discussions, now scheduled for July 30.

According to this analysis, the Soviets, who monitor the Afghans closely in Geneva, are believed unlikely to make a significant withdrawal proposal until all other issues are settled.

U.S. officials say they detect little in recent Soviet moves to suggest that Moscow is nearing a decision to pull out. "The Russians don't have much of a track record on pulling back when they move to their south," said one official.

Other officials close to the talks present a different analysis.

"Five of the six top Soviet officials who made the decision to go into Afghanistan are dead and the sixth President Andrei Gromyko has lost his influence over foreign policy," one official said.

Continuing the analysis, the official noted persistent reports of drug problems among Soviet troops in Afghanistan and of draft evasion. He also said that the mere fact Moscow permitted discussion of withdrawal was an interesting sign.

Moscow previously had insisted that Pakistan recognize the Soviet-supported government in Kabul before withdrawal discussions could start, and the removal of Babrak Karmal as Afghan leader was viewed by some, reportedly including some top Pakistani officials, as a signal that Moscow was ready to start serious discussions.

Against this backdrop, the four-year proposal is being viewed as a "cold bath" for the Pakistanis by some, but others say that the appointment of a more efficient Kabul leadership under Najibullah, the former head of Afghanistan's secret police, could be viewed as an effort by the Soviets to position themselves for a pullout.

"There are two ways to look at Najibullah," according to one analyst close to the talks.

"It simply could be that Moscow decided that the time had come for the Afghans to clean up their act," he said.

"On the other hand, a more efficient Afghan leader could be used to keep things under control while the Russians got their troops out, and then he could be replaced by someone who might stand a chance of satisfying the requirements of the neutral Afghanistan envisioned under the Geneva formula."

Under that formula, Pakistan and Afghanistan would pledge not to intervene in each other's affairs; the United States and the Soviet Union would guarantee Afghanistan's independence, including a specific Soviet pledge not to return troops, and the 2.5 million Afghan refugees in Pakistan would be allowed to return home.

Specific agreements covering these elements were reached in earlier rounds of the Geneva talks. In the latest round, agreement in principle reportedly was reached on a legal framework to encompass all these elements in treaty form, with U.S. and Soviet sanction.

U.S. and Pakistani officials use similar language in saying that six months is the outside time limit for a Soviet withdrawal and even then the majority of the troops should leave in the early stages. U.S. calculations indicate Moscow could pull its forces out in three to four months. Anything longer than that is viewed as being difficult to sell to the guerrillas.

"The Russians seem to think we can control the guerrilla forces, turning them on or off at will, but we can't exercise that kind of control," one U.S. official long familiar with the problem said.