KABUL, AFGHANISTAN, NOV. 30 -- In an offer that appeared to be timed for next week's U.S.-Soviet summit meeting, Afghan President Najibullah said today that all Soviet troops would withdraw from Afghanistan in 12 months if the United States and Pakistan cut off aid to antigovernment mujaheddin rebels.

Najibullah said the withdrawal proposal "has already been negotiated with the Soviet side." Soviet officials have suggested in recent weeks that a formal proposal for a 16-month withdrawal could be reduced to 12 months or less.

Najibullah made the announcement in a speech following his confirmation as president under a new, Islamicized constitution approved by a jirgah, or traditional conference of Afghan tribal and political leaders.

As he spoke, rebel surface-to-surface rockets pounded the western half of the Afghan capital near the meeting site. Earlier in the day, gun battles between security forces and a renegade tribal leader and his bodyguards left at least 12 people dead in the city.

{United Press International quoted government spokesman Hasmat Kahani as saying the fighting erupted when Gen. Esmatullah Muslim, a convention delegate, and his armed bodyguards defied a ban on carrying weapons into a one-mile-wide security zone of Soviet and Afghan tanks, armored cars, and troops around the convention site at Kabul Polytechnic.}

Against the background of sputtering gunfire and the drone of Soviet aircraft, Najibullah's plaintive words of peace seemed remote from this war-torn country.

The Najibullah troop proposal cuts four months off the formal Afghan offer in United Nations-sponsored peace negotiations with Pakistan, where most of the rebel groups are based and supplied. Pakistan has demanded that the estimated 100,000 Soviet troops here be withdrawn more quickly, within eight months.

In exchange, the Pakistanis would halt the supply of weapons to the rebels. The U.N.-sponsored talks offer only a formal vehicle for a settlement, without addressing the vital issue of the future government of Afghanistan.

{In Moscow, the report on the proceedings in Kabul from the official news agency Tass also indicated that Najibulah hinted at a proposal for an international conference that could help stabilize Afghanistan.

{"We are interested in conducting a high-level international conference on the normalization of the political situation around Afghanistan, with the participation of the Soviet Union and the United States," he was quoted as saying.}

Najibullah tied the withdrawal proposal and the major political reforms of recent days to the upcoming summit, including a personal appeal to President Reagan in the text of his speech.

"This is the Soviet summit position on Afghanistan," a western diplomat here said of the Najibullah offer. "This is their new timetable."

Some western diplomats based in Kabul were skeptical about the significance of the Najibullah speech. "He doesn't mention a starting date for the withdrawal period and his offer is still tied to the Americans cutting off supplies to the resistance," one of the diplomats said.

The language of the speech and of the new constitution appeared to be an attempt to woo resistance fighters back into the government and political mainstream. Most of the U.S.-financed resistance groups are tied to the Islamic faith, often on fundamentalist lines, and are therefore fiercely anticommunist.

For the past several years, the United States and its allies, chiefly Saudia Arabia, have paid nearly $600 million annually to arm the Afghan resistance against the Soviets and Soviet-backed Afghan troops. The supply of weaponry to the mujaheddin is the largest covert operation by the Central Intelligence Agency since the Vietnam war.