Diplomats ordered to leave Afghanistan before an expected rebel attack said Saturday the government will probably collapse within two months of the final Soviet withdrawal, and some said the lack of a Western presence in Kabul will aggravate the violence.

"(The government) is like a big high-rise building in which the Soviets are the girders," one U.S. diplomat said. "You take the girders out and it all comes down."The U.S. diplomat, who asked not to be named, said he expected the Soviet-supported government of President Najibul-lah to collapse within two months of the final Soviet withdrawal, expected by Feb. 15.

"The prospects of the regime coming apart are very high," the diplomat said. "You have a city full of automatic weapons just waiting to explode. It could just take a food riot."

Soviet Defense Minister Dmitry Yaz-hov met Saturday with Najibullah at the end of a two-day visit to discuss "problems of mutual interest," and assured the Afghan leader of continued arms supplies after Soviet troops leave.

"I think Yazhov was here to finalize details of the withdrawal and to say that the regime can have anything except nuclear bombs, chemical weapons or nuclear submarines," a Western diplomat said.

The diplomat said Soviet troops continued bombarding rebel positions Saturday, firing at least eight medium-range Scud missiles toward areas near the eastern city of Jalalabad.

In Islamabad, Pakistan, meanwhile, reb-els announced they had stopped attacking supply convoys on the main highway between the Soviet Union and the Afghan capital to avoid Soviet reprisals that might hurt Afghan civilians. Soviet sources confirmed hundreds of civilians died in Soviet and Afghan attacks Monday.

In announcing the decision, Burhanud-din Rabbani, an official of the Pakistan-based rebel alliance, said the Soviets threatened to continue the bombardment intended to keep the Salang highway open unless guerrillas reopened direct talks with Moscow.

Negotiations between Soviet and rebel leaders broke down earlier this month on the formation of a broad-based interim government in Afghanistan after the Soviet withdrawal.

Diplomats and Soviet sources said Moscow has extracted the bulk of the forces that invaded in 1979 to prop up the communist regime against the Moslem insurgency.

There were about 103,000 troops before last May when Moscow began a nine-month pullout under a Geneva accord signed in April by Afghanistan and Pakistan.

One Soviet source said about 5,000 soldiers in western Afghanistan were leaving their base in Shindand and most of the Kabul garrison would depart Thursday in a 100-vehicle convoy to meet the Feb. 15 deadline.

All observers expect U.S.-backed Moslem guerrillas to step up attacks on government forces after the withdrawal and uncertainty over whether the communist Kabul regime can survive has led the United States, Britain, France and Japan to order the evacuations of their diplomats. Italy and Austria joined the rush out of the country by ordering their embassies closed Saturday.

The West German mission is closed and Western aid organizations are operating with skeleton crews in snow-bound Kabul, afflicted by food and fuel shortages after rebel attacks and heavy snow inhib iting supply convoys along the Sa-lang Road leading north to Soviet territory.

The U.S. diplomat said 11 U.S. citizens, including eight diplomats, will leave the Afghan capital on an unspecified date, and he offered space on a chartered flight to any other Americans still in the capital.

At the heavily fortified U.S. Embassy in central Kabul, three armed U.S. Marines in combat fatigues guarded the lobby, but there were few apparent signs of the impending evacuation.

Raymond Petit, the French charge d'affaires, said his embassy will close Feb. 6.

"I am sorry that we have to pull down our flag at a time when Afghanistan needs the help of the entire world, when they are scared, when they need assistance after suffering 1 million dead," Petit said.

British diplomats said they would leave "sometime next week" aboard a chartered aircraft from Kabul, which is linked to the outside world by six weekly flights by Indian airlines and state-run Ariana Airlines.

He and other Western officials said they would adhere to directives from their governments, but all said they preferred to stay, warning that the absence of a Western presence could lead to unbridled brutality during the expected showdown between the adversaries.

"The Western world could become blind," one British diplomat said. "Missions prevent excesses in general, missions are arguing to stay open, but their capitals are telling them to leave. Bureaucrats and politicians like to play it safe."