Heavy shelling hit government buildings in west Beirut yesterday for the first time in a year as continued fighting at three embattled Palestinian refugee camps prevented a cease-fire from taking effect and kept the Red Cross from entering to evacuate casualties.

Unsubstantiated reports circulated in Beirut and in Europe that the civilian death toll in the camps might far exceed that already reported and that Shiite Moslem militias may be barring access to cover up the extent of the carnage.

Artillery shells, which radio reports said came from unidentified positions in the hills above Beirut, hit near the Central Bank headquarters and the office of Prime Minister Rashid Karami but caused few casualties there, witnesses said, according to Reuter.

At the Burj al Barajinah refugee camp, one of three Palestinian camps under heavy attack from Shiite Moslem militias for the past week, a Red Cross convoy was driven back by machine-gun and rifle fire from Shiite militiamen.

"They obviously don't want us to get in yet," an official of the International Committee of the Red Cross told United Press International.

The firing came despite attempts by Walid Jumblatt, leader of the Druze militia, which is allied with the Shiites in Lebanon's civil war but not in the attacks on the camps, to mediate a cease-fire to allow the Red Cross to enter the three camps, The Associated Press reported. The Red Cross has made three attempts to enter the camps since fighting erupted a week ago but has been driven back by fighting each time.

Attempts by Red Cross officials and journalists, who also have been barred, to enter the camps gained urgency today as the unconfirmed reports circulated of possible new massacres in the camps, two of which were the scenes of massacres three years ago.

The British Broadcasting Corp., the Sunday Times of London and Agence France-Presse all carried accounts of the reports, most of which accused members of Amal, the Shiite militia, of gunning down wounded Palestinians in hospitals and civilians in the camps.

In Amman, Palestine Liberation Organization Chairman Yasser Arafat told the Jordanian news agency Petra that Soviet leaders had given assurances in a letter that they would not allow the destruction of the camps and that they had been in touch with leaders of Syria, a close Amal ally, to help stop the attacks.

Officials and hospital sources put the known toll in the week of fighting in the camps at nearly 300 killed and 1,500 wounded.

The Shiite forces attacked the camps -- Burj al Barajinah, Sabra and Shatila -- in a declared effort to prevent the Palestinians from again becoming a military power in the Beirut area and in southern Lebanon, a predominantly Shiite region that Palestinian guerrillas had used as a staging ground for attacks on Israel before the Israeli invasion of Lebanon in 1982.

Concern about the rumors of wanton killings in the camps was fed by the September 1982 massacre in Sabra and Shatila of about 800 Palestinians and Lebanese Moslems by Christian militiamen.

That killing had occurred over a three-day period and became known only when the attackers left and Red Cross workers, journalists and others gained access. This time, all such outsiders remain barred from the three camps and the Shiite forces are still present.

This time, too, few western reporters remain in Beirut. Most have left because of the unresolved kidnaping in March of Terry Anderson, bureau chief of The Associated Press, and the targeting of others by Islamic Jihad, an antiwestern terrorist organization.

The Sunday Times of London said yesterday that evidence had started to mount of large-scale killings of civilians in Sabra and Shatila, and that Amal and the 6th Brigade of the Lebanese Army, a predominantly Shiite unit, were carrying out a cover-up of the operation.

Many of the published and broadcast reports yesterday centered on an alleged atrocity at the Gaza Hospital in Sabra on Wednesday.

Citing unidentified sources, the Sunday Times said that about 25 wounded patients there were killed by Amal forces and that 55 corpses were found in a truck outside the hospital, many with head wounds that appeared to have been inflicted from close range. It said Amal vehicles masquerading as Red Cross ambulances went in the camps Wednesday and removed about 100 corpses.

The BBC, citing "civilians who have managed to escape," said 25 patients "were taken away and shot last Wednesday. Wounded who could not walk were shot dead where they lay. A teen-age girl could not carry her wounded brother, so they were both shot. A nurse who went to the aid of a wounded Palestinian who had just been shot was herself bayonetted to death."

Agence France-Presse gave this account: A woman who said she had fled the hospital and was too frightened to be identified said she saw 25 Palestinians, including wounded, being shot and then buried on Wednesday. On Thursday, Hajj Mohammed, an Amal official, said about 15 Palestinian fighters had been killed in fighting in the hospital after refusing to surrender, AFP reported.

Amal, according to AFP, says it is fighting only against armed Palestinians, stressing that it has not taken any action against civilians.