The Soviet-backed government of Afghanistan, with much of its territory under rebel control, faces its stiffest challenge since it seized power 16 months ago as the result of the mutiny Sunday of an Army regiment in the capital city of Kabul, diplomatic sources said here today.

The mutiny increases the pressure on the Soviets - who have placed 1,500 military and 3,000 civilian advisers in Afhanistan and flooded the country with planeloads of some of its newest weapons - either to send in more tropps and military hardware to crush the Moslem rebels or to pull out completely.

"It's the sort of situation that doesn't look good for the Soviets," said one U.S. expert.

"They are ruhning a lot of risk there," he continued.

On the one hand, observers said, if the Soviets get more deeply involved in the fighting they will invite reaction from Moslems in their own country and in the neighboring nations of Iran and Pakistan, who already are being accused of taking part in the revolt.

On the other hand, if the Soviets do not give the Afghan government of Nur Mohammed Taraki added support, they may be accused of abandoning an ally in much the same way the United States was blamed for not aiding Iran and Vietnam.

Until recently, the rebellion of uncoordinated groups of Moslem tribesmen has been going on in the rugged mountains and steep valleys of the Afghan countryside.

During the last month, however, the fighting has spread to main highways, and in late June there was a small battle in the marketplace of Kabul.

But Sunday's mutiny was the biggest battle fought in Kabul, and the first time heavy Soviet weapons were used there.

As a result of the increasing pressures on Kabul, the United States ordered all dependents of embassy personnel out of the country last month. They had all left by Sunday, the State Department reported.

The Afghan government was forced to use the full weight of its Soviet-supplied arms to crush the mutiny in the four-hour battle, travelers from Kabul told the Associated Press in New Delhi.

According to the AP report, which could not be confirmed by the State Department here, the mutiny started about noon Sunday when three tanks rolled down from the hilltop fort of Bala Hissar in Kabul toward the former royal palace, where Taraki was supposed to be holding a Cabinet meeting.

The tanks were reported to have been destroyed by three rocket-firing MI24 helicopter gunships - highly sophisticated Soviet weapons that U.S. officials believe cannot be flown by Afghan pilots.

The AP also reported that conversations between at least two of the helicopter gunship pilots and their base were conducted in Russian, adding to speculation that the Soviets had taken a direct role in the fighting.

After destroying the tanks, according to reports reaching New Delhi, the gunships turned their rockets for the next 2 1/2 hours on the historic Bala Hissar fort, which was surrounded by 16 tanks.

All the while, Soviet-made jets streaked over the city but took no part in the fighting, according to observers.

U.S. diplomats watched the battle from the embassy, but Afghan troops warned them not to go on the roof for a better view.

This was the third army mutiny since March - when some troops joined a civilian uprising in the city of Herat - Afghanistan's Afhanistan's border with the Soviet Union and Iran - that saw Soviet advisers singled out for killing.

More recently, a garrison in the city of Jalalabad was reported to have mutinied.

But Sunday's fighting showed that elite armored and commando units in Kabul remained loyal to Taraki's government, observers here said, even though morale among troops stationed in outlying provinces is believed to be low, with defections of whole units to the rebel side being reported increasingly in recent weeks.

Although the rebels control much of the countryside, they are believed to have no chance of taking over the country as long as the government holds Kabul.

The rebels' inability to coordinate their fight has helped the government keep its control of the country. For the most part, the rebels are Moslem tribesmen fighting what they feel is "godless Communism" that will keep them from practicing their religion.

But in the tradition of the fiercely independent tribesmen of the country, they also are resisting Taraki's efforts to impose central control over Afghanistan's 28 provinces and his attempts to institute land reform and changes in marriage and dowery laws.