Fighting between Soviet troops and Afghan Moslem insurgents is said to be continuing in several provinces in eastern Afghanistan, with unconfirmed reports that a provincial capital has fallen to the insurgents.

Diplomatic sources said they had received reports that the city of Taliqan had been captured within the last few days from beleaguered Afghan government troops. The Soviet invasion force that began pouring across the border two weeks ago was not believed to have moved into that city. With more than 66,000 people, Taliqan is the capital of the northeastern Afghanistan province of Takhar, which borders the Soviet Union.

Last night, a convoy of Soviet troops could be seen driving from a base near Kabul to the airport, where transport planes or helicopters were heard readying to take off. The Soviet troops appeared to number about 400. Most appeared to be of Tajik origin from that region of the Soviet Union near the border with Afghanistan. The Soviets' 27-truck convoy, followed to the airport by reporters, included nine trucks loaded with equipment.

It could not be determined where the troops were going. Observers speculated that they were being airlifted to one of a number of trouble spots in eastern Afghanistan.

The nation's new president, Babrak Karmal, told a news conference "the small-numbered Soviet military contingent" will leave the country as soon as "intervention" by foreign powers supporting Moslem Afghan rebels is over, the Hungarian news agency MTI quoted by the Associated Press as reporting. Karmal claimed the insurgency is supported by the United States, China, Pakistan, Saudi Arabia and Egypt.

MTI quoted Karmal as saying Afghanistan's revolutionary council had asked the Soviet Union for assistance even before Dec. 27, when the Soviets sent thousands or troops into Afghanistan and helped install Karmal in place of Hafizullah Amin, who subsequently was reported executed.

Others eastern provinces in which fighting has been reported in the past few days include Badakhshan, Laghman, Baghlan, Paktia, Nangarhar and Ghazni.

"The most active insurgency continues to be in Badakhshan and Takhar provinces," a diplomat said. Another reported that Paktia was equally turbulent, and and that the insurgents to havens in Pakistan.

Diplomats said it has been impossible so far to verify reports of casualties on both sides or determining the extent and intensity of the fighting.

Before the Dec. 27 coup, Badakhadhan province was reported to be almost completely in the hands of the insurgents except for the capital, Faizabad. Harried Afghan government troops were Barely retaining control there. Since the coup, Soviet troops are said to have gone to Badakhanhan, but there has been no evidence that they have succeeded in overrunning either it or Takhar Province.

However, with two more Soviet divisions reportedly joining the five already in the country and bringing total Soviet strength here to an estimated 85,000 men, diplomats here feel it is only a matter of time before the Soviet troops enter the troubled provinces in force.

"They are going there to finish off the rebels," an Asian diplomat said. But he added that, based on more than a decade of experience here, he believed the Afghans would continue to fight what they regard as a war between Islam and communism.

"There will be no peace in this country for quite some time," he said. "For the Russians, it will be Vietnam ultimately."

The Soviets so far are reported to control all major cities and towns and key military installations.These include Kabul, Kunduz, Mazar-Sharif, Bagram airbase, Jalalaoad Kandahar Shindand airbase and Herat.

Soviet troops are becoming less and less visible in the Capital but elements of an airborne and a motorized rifle division reportedly totaling about 16,000 men are dug in around the city.

The Soviets continue to airlift supplies into Kabul, landing an average of about a dozen transport planes a day at Kabul airport..

In many areas the Soviets are reportedly still in the process of putting themselves into position. In the southern provincial capital of Kandahar, according to diplomatic sources. Soviet troops recently moved into newly built Army barracks. The Afghan soldiers already living there were moved elsewhere.

Moscow's ambassador here has been mostly silent about the Soviet invasion. But in one contact with Western ambassador shortly after it, he claimed that Soviet troops had come to Kabul only to protect foreign embassies and maintain order. Asked elsewhere in the country, he said they were there to protect Soviet economic aid projects.

Meanwhile, the plight of Afghan regugees on the Pakistani side of the border is said here to be growing increasingly serious. According to one diplomatic report, the number of refugees had grown recently from about 400,000 to 500,000 and may soon reach 700,000.

The report said mountain passes between the two countries are likely to be snowed in within 30 days, resulting in large loss of life for refugees trying to flee the fighting in the Afghan border provinces.

Afghan refugees in Pakistan's North-West Frontier Province are becoming increasingly restive, according to the report, and may come into conflict with local Pakistanis if additional relief is not supplied.