The senior envoy from the closed U.S. Embassy in Afghanistan said Wednesday that most Western and neutral diplomats in Kabul believe President Najib's government, divested of Soviet military support, will fall by July.

"The regime in many senses is like a building without its girders," Jon D. Glassman told a news conference the day after the last 11 U.S. Embassy staff members left the Afghan capital."I think there's a general consensus among most informed observers that the life of the regime is likely to be short," said Glassman, who served as charge d'affaires at the embassy for the last 18 months. "There's no support there (among the people)."

Only about 15,000 to 20,000 Soviets soldiers remain in Afghanistan, according to Western and Soviet diplomats, and they are to be gone by Feb. 15 under an accord sponsored by the United Nations.

When asked how long Najib's forces can hold out against Moslem guerrillas ensconced in the snow-covered mountains surrounding Kabul, Glassman said: "This is a work for a crystal ball." But he added: "In Kabul, most of the Western and neutral diplomats are betting between now and July."

The United States closed its embassy on Monday, citing fears that security will deteriorate because of the Soviet departure. The West German Embassy shut down Saturday. Several other non-communists nations are expected to follow suit within a few days, including Austria, Britain, France, Italy and Japan.

India, one of the few non Soviet-bloc nations to maintain full diplomatic relations with Kabul since Moscow's intervention, this week evacuated families of its 75 embassy staff members.

Most Soviet bloc nations have also sharply scaled down staff.

Glassman rejected Afghan accusations that the closing of the U.S. Embassy and Washington's concern about security were a "psychological ploy" to undermine Najib.

"I think the evolution of events are foreordained already by 10 years of war, 10 years of foreign intervention and the people's objection to that."

He said anyone looking for "physical evidence" that Najib's government is teetering need only look to the Salang Highway, Kabul's food and fuel lifeline to the Soviet Union, and Afghanistan's fertile northern plains.