Soviet forces, apparently in full control of Afghanistan's capital city, today were reported to be moving into the mountainous provinces where rebelling Moslem tribesmen are posing their greatest threat to Marxist government.

According to diplomatic sources here, units of Soviet troops fanned out by air during the last two days into regions along Afghanistan's eastern border with Pakistan.

The first group of Western journalists to arrive here since the Soviet-led overthrow of Hafizullah Amin's government three days ago were told by residents of Kabul that Soviet troops were continuing to play a major security role in the city, with Afghan troops relegated to support roles or kept in their barracks.

After being detained for seven hours at the airport, which reopened only today, I and nearly all of the arriving journalists were ordered to leave the country. During that time, however, we were able to speak with diplomats and Afghans.

News services, quoting Western diplomats in the region, reported that the Soviet Union poured thousands of fresh troops and military equipment into Afghanistan today, most of it along the border areas where rebel forces are concentrated.

An independent Pakistani newspaper, Jung, reported that the newly arrived Soviet troops, fighting side-by-side with Afghan soldiers against the Moslem guerrillas, had regained control of northeast Takhar Province bordering the Soviet Union in a fierce, eight-hour battle.

The rebels, according to the Jung account, carried by United Press International, said they still controlled outlying areas of the province. They said they suffered 400 casualties, but that they wiped out an entire mixed brigade of Soviet and Kabul governmen troops.

Since the overthrow of Amin and the installation of President Babrak Karmal, a Marxist considered more loyal to the Kremlin than was Amin, communications have been largely cut off in Afghanistan and with the outside and travel has been restricted inside the country.

The diplomats interviewed at the airport here acknowledged that their information was largely tentative and unconfirmed.

But they said usually reliable sources in Kabul had informed them that it appears the Soviet combat forces there, estimated by the White House to be as many as 25,000 troops, would take a stronger role from now on in fighting the rebels, who have bedevilled the government for the past year and left it controlling only the major cities and highways.

Up until now, Soviets were not believed to have taken a combat role in the fight against the rebels, restricting their activities to advising the Afghan army, which did the actual fighting.

The Soviet presence was easily seen during seven hours spent at the airport today, and diplomats and other residents of Kabul confirmed reports that it was Russian troops -- not Afghanis -- who had control of the city.

"Welcome to occupied Afghanistan," said one resident of Kabul with a sad smile.

Despite the strong Soviet presence here, a representative of the Afghan Ministry of Interior explained today that the government could not yet allow journalists into the country because "the present situation is not good."

"The government cannot guarantee your safety because of the existence of Amin's terrorist bands."

While most observers here felt the present government -- backed by the armed might of the Soviets -- was in full control of Kabul, they reported that volleys of gunshots are heard here every night.

"We always hear them shooting in Kabul," said one woman, "but there's been more of it since the Russians moved in Thursday night."

The massive airlift of Soviet armaments and troops has apparently stopped coming in at Kabul's international airport, although it may be continued at three military airfields the Soviets control.

But the airport here is still filled with Russian equipment. Six grant Antonov transport planes were lined up this morning on the apron, and hundreds of tanks and armored personnel carriers stretched alongside the runway for about three-fourths of its length.

In one corner of the airport, the Soviets have set up a tent city and a detachment of Russian soldiers marched past the airport arrival lounge with one man carrying a small Christmas tree.

According to diplomats, Soviet troops in Afghanistan were a mixed unit of central Asians, who resemble the Afghans, and Europeans.

One transport plane landed this afternoon, and shortly after it taxied out of sight of the passenger lounge, where the reporters were detained, a convoy of military trucks was seen speeding up the runway.

Travelers at the airport said they had noticed an unusual amount of Soviet troop movement in Kabul today.

The city itself is shut off from most of the outside world and from the rest of Afghanistan. The first flight since Thursday night's coup arrived here today. No telephones work in Kabul nor is there telephone or telex communication with the outside world -- apparently as a result of an explosion outside the main telephone-telegraph office in downtown Kabul during the coup. o

While the radio and television station appears undamaged, there have been no television broadcasts since the coup.

This is one reason why Afghanistan's new president, Karmal, has not been seen since he took office. In the last two coups of the past 19 months, almost the first order of business was for the new president to give a major television speech.

The lack of a public appearance by Karmal has led to speculation here that he was not in Afghanistan when the government of Hafizullah Amin was overthrown and he was named the new president.

Karmal was a deputy prime minister in the government of former president Nur Mohammed Taraki. He was later exiled as ambassador to Czechslovakia, but remained in Eastern Europe after he was ordered back during a purge of leaders of his Parcham (Flag) wing of the pro-Soviet Afghan People's Democratic Party.

There were rumors today that Karmal had not arrived in Kabul until Saturday, although most observers believe the Soviets sneaked him in last week when they were airlifting massive numbers of troops on the pretense of helping to keep the Amin government in power by fighting the rebels.

Instead, these troops were used a few days later to overthrow the Amin regime.

According to one diplomat here, Afghan tank units took no part in the fighting -- possibly, according to one rumor here, because all their ammunition had been taken from them on the day of the coup on the pretext that an upcoming training exercise required them to use blank shells.

"It's just awful," said one woman waiting for a plane out of the country. "There are too many Russians. The streets are full of Russians."

Asked another Afghan, "Would you like it if someone came into your country and imposed a new government?"