The resumption of U.N.-sponsored peace negotiations in Geneva and observance of the Islamic fasting period of Ramadan have touched off the heaviest fighting by Afghan rebels in and around Kabul in a year, according to western intelligence reports from Afghanistan.

Diplomatic sources here said today that Afghan guerrillas launched four major operations against Soviet and Afghan forces near Kabul, the capital, using rockets, mortar and machine-gun fire. The raids were followed by extensive bombing by Soviet and Afghan aircraft.

In some areas near the capital, the fighting was the most intense in two years, according to reports from two western diplomatic missions in Kabul.

The intelligence assessments linked the stepped-up rebel activity to the resumption of the indirect negotiations in Geneva for withdrawal of Soviet troops from Afghanistan and the June 11 beginning of Ramadan.

They said that on the night of June 11 Afghan guerrillas staged a three-hour assault on a Soviet command headquarters behind the Darulaman Palace in Kabul, and that the next night three rebel groups with mortars and rockets attacked the Bala Hisar fortress housing Soviet forces on the eastern ridge of the Sherdawaza Mountains.

There were no reports of casualties, but diplomatic sources said Soviet helicopter gunships bombed the guerrilla positions. They said the guerrillas appeared to be well-trained and froze in position when illumination flares were dropped so that movement could not be detected. The intelligence reports said guerrillas dressed in Afghan Army uniforms caused confusion among the fortress' defenders, and that a large number of Soviet and Afghan armored vehicles were destroyed.

The night of June 13, three Afghan Army posts near the village of Shewaki, just south of Kabul, were attacked for four hours, and a reinforcement unit of 300 Afghan troops suffered heavy casualties, the reports said.

That night marked the "most intense and prolonged fighting in or around Kabul this year," according to one western intelligence report.

In the Karte Se area of the capital, three police posts were attacked by rebels and at least one was captured while Soviet armored personnel carriers raced around in apparent confusion, diplomatic sources said. They said an Army outpost at nearby Khan Khana was attacked and weapons and ammunition were seized, along with some prisoners.

The raids were viewed as giving a boost to the guerrillas, who, although able to score some tactical victories over Soviet and Afghan troops recently, had failed to mount a successful offensive this year.

However, Soviet artillery batteries retaliated on June 15 with a six-hour bombardment of Ghaza in the Shomali Valley, four miles northeast of the former resort town of Paghman, diplomatic sources said. They said civilian casualties in the town were high, but they gave no figures.

Saturday, Soviet helicopter gunships bombed villages in the Paghman area suspected of harboring guerrillas, and increased Soviet MiG fighter-bomber activity was reported at Kabul's airport.