U.S. diplomats lowered the Stars and Stripes and closed their mission in embattled Kabul Monday as departing Soviet troops headed north in what one Moscow envoy said could be the last truck convoy home.

Afghan troops and Moslem guerrillas continue to fight for control of the Salang Highway, the only road linking the capital and the Soviet border.Insurgents based in Pakistan said Sunday that Soviet bombing and missile attacks along the 260-mile highway had killed 600 civilians and injured 1,200 in the past several days.

"The mangled bodies are still under the debris," the Afghan News Agency, operated from bordering Pakistan by a guerrilla group, said in a telexed statement.

Official Radio Kabul, monitored in Pakistan, said Afghan troops had carried out "successful military operations" with civilians who want to keep the rugged Salang Highway open.

Later in the day, a car bomb and a truck bomb injured several people in the capital, a Foreign Ministry spokesman said on condition of anonymity.

"Not many" people were injured, he said, but declined to give the exact number or say exactly where the explosions occurred.

Just north of Kabul on Sunday, a convoy of Soviet soldiers in armored personnel carriers and trucks loaded with missile launchers cruised through a Soviet bunker checkpoint as helicopter gunships hovered nearby, offering cover.

A Soviet diplomat, speaking on condition of anonymity, said, "This could be the last convoy." He refused to elaborate.

Two fighter jets streaked overhead and disappeared behind the snow-covered mountains just beyond the hills surrounding Kabul.

The remaining Red Army troops apparently are leaving on transport planes, completing a withdrawal begun eight months ago under a U.N.-brokered agreement to end the Soviet's nine-year intervention in Afghanistan.

Moscow sent about 115,000 troops into Afghanistan in December 1979 to prop up Marxists who seized power in a coup the previous year.

Soviet diplomats in Kabul say the remaining 15,000 to 20,000 Red Army troops could be gone by the end of the week, about 10 days before the U.N. deadline of Feb. 15.

The guerrillas are not a party to the U.N. agreement and have vowed to overthrow the government of President Najib once the Soviets are gone.

At the U.S. Embassy in Kabul, American diplomats said they would return once peace is restored in this war-torn country.

"Today we leave at a moment that is both happy and sad," said the charge d'affaires, Jon D. Glassman, after a flag-lowering ceremony in the embassy's snow-covered compound.> "We are pleased that the people of Afghanistan are going to be relieved of their suffering, but we know their struggle is not over. The people of the United States are with them," he said. "We will be back as soon as the conflict is over."

At the ceremony, three Marine guards took down the U.S. flag, fold-ed it and handed it to Glassman.

The State Department last week ordered the 11-member embassy staff to leave, citing fears the Afghan army may not be capable of protecting foreign diplomats once the Soviets leave.

West Germany, Britain, Japan, France and Austria all have withdrawn or are withdrawing their embassy staffs. Italy and some of Moscow's East Bloc allies are considering similar action.

Glassman declined to say when the U.S. staff would leave Kabul aboard a chartered flight to India. The airport was closed Monday due to snow and fog.> After the embassy staff departs, 10 Americans will remain in Kabul. All are missionaries.

The Afghan government has denounced the embassy closures as unnecessary and aimed solely at encouraging the U.S.-backed Moslem insurgents to intensify their fighting.> At a Soviet checkpoint about 12 miles north of Kabul on Sunday, Afghan soldiers paced nervously along the Salang highway as the Soviet convoy rolled past.

"How's everything going?" one Afghan officer was asked.

"Very bad," he said.