For many weeks, the feeling had been growing among South Asia correspondents based here and in neighboring Pakistan that some new, sinister and destructive event had been unfolding behind the forbidding borders of Soviet-occupied Afghanistan.

Afghan travelers arriving in New Delhi on the almost daily flights from Kabul, the Afghan capital, have spoken vaguely of Soviet massacres of civilians suspected of assisting Moslem guerrillas fighting the Soviet and Afghan armies.

The Afghan exile community here and along Pakistan's western border also had been abuzz with reports of large numbers of civilian deaths, although, as often is the case, corroborative details have been scarce.

But accounts of these massacres, in which hundreds of old men, women and children were reported killed in Afghanistan's northeastern province of Laghman in early March, attributed to anonymous western diplomatic sources, did not appear in the world press until several days ago.

As Afghanistan's shadowy war has dragged on through five years, these "western diplomatic sources" have taken on increasing importance in the battle for public opinion between the Soviet Union and the West.

The Afghan government does not permit western reporters to visit the battle zones to verify the reports independently. Periodically, correspondents slip across the porous Pakistan-Afghanistan frontier with bands of insurgents, but because of the constraints on travel imposed by the rugged terrain and the danger from a growing number of Soviet and Afghan Army troops, their reports are often limited to a relatively narrow geographical area or to a limited time period.

As a result, the weekly briefings held here and in Islamabad by the embassies of western nations that have retained diplomatic missions in the Afghan capital of Kabul have evolved into an apparent attempt to fill the information gap.

At midday almost every Tuesday, a half dozen foreign correspondents based in Islamabad gather in a windowless room on the ground floor of a western embassy building to hear an account of the war in Afghanistan between the Soviet-backed communist government and the Moslem insurgents.

An embassy spokesman, who has not been to Afghanistan and who frequently stumbles over the pronunciation of the names of unfamiliar towns, begins reading in a monotone from a long, teletyped cable from his country's mission in Kabul.

The spokesman does not have to spell out the ground rules for the briefing, which are well known: notes can be taken but tape recorders are not permitted nor will copies of the document be distributed.

Attribution is strictly limited to "western diplomatic sources," even though the Soviets and the Afghan government are well aware of the regular briefing sessions and can obviously see through the thinly disguised sourcing that appears in the next day's newspapers. The diplomatic sources readily concede that there is little doubt their cables have been intercepted by Soviet intelligence.

At the same time as the Tuesday briefing in Islamabad, foreign journalists in New Delhi gather around a conference table in the chancery of a western embassy and listen to a recitation of an identical report on the guerrilla war. As in Islamabad, substantive questions are not entertained, because the briefing officer has no information beyond that contained in the written report.

Usually, the New Delhi-based correspondents then go to the nearby chancery of another western diplomatic mission and listen to the reading of a similar war report, whose details often -- but not always -- mirror those of the first embassy.

Last week, the diplomats reported that the massacres occurred between March 11 and 18, more than a month after the time reported by the Afghan Information Center, which disseminates reports from insurgent groups and other sources about the war in Afghanistan. The center is based in Peshawar along Pakistan's western border and is in regular contact with correspondents based in Islamabad and New Delhi.

Citing accounts by refugees arriving in Peshawar, the center said that in one village alone, Kats, 650 civilians died, and that there were other massacres in Logar, Paktia, Nangarhar and Konarha provinces, all close to the Pakistani border where there are heavy concentrations of Soviet troops.

The diplomats said that details of the incidents had been received from survivors only recently.

This week, two embassies in New Delhi offered briefings on the Afghan war, but neither mentioned the massacres. Instead, minute detail was presented about a recent series of car bombings and other guerrilla attacks in Kabul, underscoring the sense here that reports of war developments inside Kabul, where western diplomats are based and have first-hand information, are generally more reliable than those originating in remote areas of the country, from which almost all information reaching Kabul or elsewhere is second- or third-hand.

This difference in access to reliable accounts of combat has led to a suspicion of casualty figures supplied by the western diplomats in Kabul.

In November 1982, western diplomatic sources here reported a major disaster in the Salang tunnel through the Hindu Kush mountains in northern Afghanistan, in which witnesses were quoted as saying that more than 700 Soviet soldiers and 400 Afghan civilians had died in an explosion, and that as many as 2,700 persons might have been killed.

The story, emphasizing the death toll, received wide attention in the international press. A month later, the same western diplomatic sources scaled down the casualty total to 350. The second account by the diplomatic sources also contradicted earlier reports by the embassy spokesmen that Soviet soldiers had prevented civilians from leaving the tunnel, resulting in further deaths by asphyxiation.