MOSCOW, JAN. 7 -- With the visit of Foreign Minister Eduard Shevardnadze to Kabul this week, the Soviet Union has stepped up the pressure on the ruling communist party in Afghanistan for an early settlement of the eight-year-old Afghan war and intensified a major diplomatic effort to end fighting there, western diplomats said.

In an interview released yesterday in Kabul, Shevardnadze told the official Afghan news agency that the withdrawal of 120,000 Soviet troops there should begin this year. "We would like the year 1988 to be the last year of the stay of Soviet troops in your country," he said in the interview.

One of the main goals of Shevardnadze's trip, observers here said, was to convince Afghan President Najibullah of Moscow's determination to end its military involvement in Afghanistan, even if it comes before a political settlement is found. In recent months Najibullah has shown signs of resisting Moscow's efforts to carry out an early withdrawal of its forces.

"The whole trip was to put more heat on Kabul," said one diplomat. "They have really been dragging their feet on the pullout timetable."

In a clear rebuke to Najibullah's efforts to secure his own political survival, Shevardnadze stressed that it is indispensable that "no one . . . claim the monopoly to power" in Afghanistan and warned that those who put "some transient, circumstantial, personal considerations and aspirations above the interests of the nation" are blocking reconciliation.

Western diplomats here view the Soviet foreign minister's mission as the most crucial aspect of a major Kremlin effort to reach a consensus for ending Soviet involvement in the war. Aside from Shevardnadze's meetings with Najibullah, senior Soviet officials are reportedly holding talks on ending the war with other interested parties, including the Reagan administration and some of the Afghan rebel groups fighting the Soviet-backed regime.

The Soviet diplomatic effort is expected to climax in February, with United Nations-sponsored talks between Pakistan and Afghanistan in Geneva. Moscow is likely to publicize its program for troop withdrawals; and an announcement of an agreement could come by early March, with a troop pullout beginning in May, according to diplomatic sources.

Moscow already is seeking ways to expedite the troop withdrawal. The United States has agreed to act as a guarantor for ending of hostilities in Afghanistan, the Soviet Foreign Ministry said today. This role commits Washington to stop its aid to Afghan rebels after Soviet troops begin to leave.

"By taking private assurances and making them public, {the Soviets} are being more concrete about their own intentions," noted one western diplomat here who closely follows developments in Afghanistan.

Shevardnadze's public statements, made as he was leaving Kabul, seemed directed at two of the main players in the Afghan conflict -- the Soviet-backed Afghan government and the United States, chief arms supplier to the rebel forces.

By highlighting the U.S. role in an Afghan peace, Shevardnadze may be trying to force the Reagan administration to deal with any political problems posed by an agreement to cut off aid to the mujaheddin, or Moslem "holy warriors," diplomats here noted.

This may have added an extra measure of urgency to the Soviet timetable for Afghanistan, already driven by the domestic and international pressures created by the war. Most analysts here think Moscow already has decided that the war is unwinnable militarily, despite the recent offensive near the eastern town of Khost. For more than a year, Soviet leader Mikhail Gorbachev has described the long-term solution as an "independent, sovereign, neutral and nonaligned Afghanistan."

Western diplomats here say Moscow's offers to cut back on the 12-month withdrawal timetable, already a sharp reduction from the original 48-month proposal, have been resisted by Kabul.

The faster Soviet troops leave Afghanistan, the more vulnerable will be Najibullah and his ruling People's Democratic Party of Afghanistan (PDPA) during the violent period expected after the pullout. Rebel groups fighting the Moscow-backed regime have refused to enter into any coalition government with Najibullah, whom they regard as a Soviet puppet.

In spite of any qualms on Najibullah's part, Moscow seems determined to prevent the timetable from blocking negotiations. Some diplomats note that the Soviets were irritated last fall, during the last round of Geneva negotiations, when Afghanistan did not bring its proposal for a withdrawal schedule down to one year, as Moscow had been hinting it would do.

The 12-month timetable, only four months away from Pakistan's demand for an eight-month withdrawal, emerged as the official Afghan government position in December at a convention to approve a new constitution.

While Shevardnadze was in Kabul talking to Najibullah this week, other Soviet diplomats were making public statements that the new 12-month timetable could be reduced even further. That was reasserted today at a press briefing at the Soviet by Yuri Alexeyev, the Foreign Ministry's regional deputy. "The point at issue is months, not years," he said.

According to diplomats here, other key issues remain to be decided on the withdrawal issue at the Geneva talks, including the scheduled phases for the departure of the Soviet troops and the monitoring mechanism. According to Shevardnadze, the withdrawal would begin 60 days after the signing of an Afghan-Pakistani accord, although some here say that figure may still be negotiable.

"The most important thing now is to conclude the Geneva process," said Alexeyev. "A pattern of settlement has nearly been agreed upon." Shevardnadze said that after his talks, he and Najibullah concluded that the conditions for a withdrawal can be ensured "shortly."

Shevardnadze also hinted that the ruling PDPA will have to find its own political solution: the formation of a "coalition government on the broadest of basis."

At the Foreign Ministry today, Alexeyev called the PDPA just one of several Afghan political parties contending for power. But he also indicated that the Soviet government is not prepared to let the PDPA be overwhelmed by other factions.

"Now the PDPA is only one of the political parties," said Alexeyev. "This party does not claim monopoly, but it does not mean it will resign from the political scene of the country."