KABUL, AFGHANISTAN, NOV. 29 -- Four rebel rockets slammed into a Kabul hillside today near a meeting where Afghan leader Najibullah was addressing progovernment tribal leaders and politicians in an assembly that will elect a president and approve a new constitution.

Later, an obviously angry Najibullah, leader of the ruling People's Democratic Party, lashed out at the "blind firing and loud noises staged by the mercenaries of imperialism."

In a significant confirmation of recent combat gains by the anticommunist mujaheddin rebels, Najibullah said they have cut off supplies to the city of Khost, 100 miles south of Kabul. He said that, because of the effectiveness of U.S.- and British-supplied ground-to-air missiles used by the rebels, the government has been unable to supply the 40,000 residents of Khost by air during the day.

He also said that roads to Khost are being blockaded. "We cannot supply the principal material needs of Khost," he said.

Until recently, the government had described Khost as a totally secured "zone of peace."

There is abundant evidence in Kabul in recent days that Soviet and Afghan government forces are mounting a counteroffensive against resistance positions in the Safed Koh Mountains surrounding Khost. For the first time in the seven-year war, in which as many as 1 million people may have died, Soviet SU25 fighter bombers are making regular runs from Kabul airport.

"They are going every 20 minutes with bombs under their wings and coming back empty," a western diplomat said.

In the past two weeks, several large Soviet truck and armored vehicle convoys have been seen leaving Kabul, including a 300-vehicle convoy yesterday. Speculation at western embassies here is that all were headed to Khost to supply forces attempting to break the siege.

Today's rocket explosions shook the auditorium at Polytechnical University, where 1,430 delegates to a traditional Afghan "grand assembly" had gathered to pick a president and approve a new constitution.

One witness said the rockets exploded on an unpopulated hillside less than a mile from the meeting site. The university campus was surrounded by Soviet tanks and armored personnel carriers.

After the explosions, Soviet security men rushed to the side of the Soviet ambassador attending the meeting. Soviet Mi2 helicopter gunships circled the campus.