The Soviet-backed Afghan government continued to maintain secrecy today on the reported tunnel disaster between the Soviet border and Kabul, despite new and conflicting accounts of the cause and extent of it.

Afghan exile sources here, monitoring Kabul radio, said that government broadcasts had stopped mentioning funeral arrangements for victims of the disaster, in which hundreds of Soviet soldiers and Afghan civilians are believed to have been asphyxiated following the collision of a fuel tanker truck and the lead vehicle of a Soviet Army convoy. Kabul has been described by Western diplomats there as "a city in mourning."

Neither the Marxist government nor the Soviet military command has even acknowledged that a disaster in the Salang tunnel, 60 miles north of Kabul, occurred -- leading Afghan exiles and Western diplomatic analysts to conclude that casualties could be as high as originally feared. Otherwise, the government would be expected to issue strident denials, as it usually does following erroneous or exaggerated accounts of casualties or damage inflicted upon state security forces.

The initial reports that emerged Tuesday from one Western diplomatic mission in Kabul said that 700 Soviet soldiers and 400 Afghan civilians perished in an explosion and fire in the 1.7-mile-long tunnel. The diplomatic sources cited "unconfirmed rumors" that the civilian deaths could be as high as 2,000.

However, diplomatic sources in another Western embassy here, who did not not even mention the Salang Pass incident to reporters during a regular briefing on Afghanistan Tuesday, now say that 50 Soviet soldiers and between 500 and 1,000 Afghan civilians apparently died in the tunnel disaster, which is said to have occurred Nov. 2 or 3.

Recently arrived travelers from Kabul told Afghan exiles here that 400 bodies had been brought from the tunnel to Kabul.

"It is obvious that something big happened in that tunnel, but we don't know exactly what," a Western diplomat said. An Afghan exile here, who is usually well informed about events in his country, said today, "It is all very confusing. I toned down my estimate to 400 dead, but now I just don't know what the figure should be."

Afghans claiming to have witnessed the disaster gave reporters in the Pakistani border city of Peshawar widely varying casualty estimates. Some said they saw six trucks loaded with bodies leave the scene, and others said there were only three trucks.

There were also many unanswered questions about the cause of the disaster, although most versions agreed that the victims suffocated in toxic fumes after Soviet security forces, apparently believing a guerrilla attack had taken place, were said to have blocked both entrances of the tunnel and prevented vehicles from leaving.

Rebels in Peshawar at first denied any involvement in the disaster, saying their guerrillas could have launched an attack against the tunnel anytime, but never did because of the obvious risk to civilians.

But one rebel group, the Gulbiddin Hekmetyar faction of the Islamic Party, yesterday claimed in Peshawar that it had mined the tunnel and its approaches. "We killed 600 to 700 Russians. Unfortunately, there were some civilian casualties," spokesman Mangal Hussein said, according to news agency reports.

Agency Afghan Press, a Rawalpindi-based organization that monitors the insurgency in Afghanistan, said it interviewed the guerrilla commander who set the explosive charges. The agency said the tunnel had collapsed in three places.

However, other guerrilla sources in Peshawar were quoted as saying a Soviet convoy turned back to the Salang tunnel after being ambushed in the Panjsher Valley. Mohammed Yaqub Sharafat of the rival Yunis Khalis guerrilla organization told reporters in Peshawar that the lead vehicle then collided with a tanker.

Some Afghan nationalists in exile expressed skepticism concerning the sabotage claims by the Gulbiddin Hekmetyar, noting that it is distrusted by many other guerrilla groups. They said Hekmetyar, who has lived in Pakistan since his release from an Afghan prison in 1973, is suspected of kidnaping rival guerrillas opposed to his rigid fundamentalist leadership.

Apparently embellished accounts of the disaster, laced with vivid descriptions of daring exploits, continue to sweep through the Afghan exile community, reinforcing a reputation for legend making. One prominent exile here spoke emotionally of a "well-planned suicide mission," and then admitted that his account was conjecture.