After three months of fighting in Afghanistan, Soviet forces are still coping with a widespread insurgency rather than crushing it with overwhelming force, according to U.S. official assessments.

Periodic search-and-destroy operations using armor, airpower and other conventional weapons and tactics have taken a steady toll in casualties among both Soviet forces and Afghanistan rebels, Washington analysts said.But as in American operations in Vietnam a decade ago, the overwhelming conventional fire-power is only temporarily effective against local insurgents, who return to the area when the foreign forces withdraw to their base camps.

The latest assessment of the Afghan situation, provided in a press briefing yesterday by analysts who declined to be identified by name or agency, was notable for what was not reported.

Despite considerable speculation to the contrary in the early days of the Afghanistan invasion, the Soviet Union shows no sign of a further large-scale augmentation of its battlefield units, according to the U.S. officials. Neither the Soviet expeditionary force of about 80,000 troops nor the border area reserve force of about 25,000 troops in nearby areas of the Soviet Union has been expanded in the past month, the officials said. And it appears, from this perspective, that the current force is far from sufficient to take the offensive nationwide against 50,000 to 100,000 lightly armed Afghan insurgents.

The Soviet Union is "in a dilemma about taking a bigger role," according to a U.S. analyst. A State Department source suggested that the Russians may seek to make gains with their present forces over the next several months before deciding whether to assume the greater risks and costs of expanded involvement.

According to the analysts, there is yet no sign that the Soviet-installed government of President Babrak Karmal has made headway in expanding its political base of support or acceptability among the Afghan populace. As indications of this, they cited reports of continuing infighting among communist party groups, the inability of the Babrak government to recruit respected noncommunist supporters, and the inability of the ruling group to agree on a new national flag more acceptable than the present red banner.

The Babrak regime several months ago disclosed plans to change the unpopular red flag, which had been adopted in late 1978 by a predecessor communist government. Last Wednesday, according to press reports, Babrak announced that the country will henceforth have two flags -- the red flag symbolizing the communist party and a red, black and green flag symbolizing "independence and freedom." The U.S. analysts said they were impressed by both the lengthy nature of the deliberation and the inability to agree on a single banner.

Another internal problem that augurs continuing difficulty for the Russians, according to the U.S. sources, is the inability to reverse the decline in the Afghan military force. A major Afghan conscription campaign has run into trouble, the sources said, and some Afghan army units continue to defect.

A gain for the Russians in the Afghan situation, as depicted by the U.S. analysts, is the decline in overt insurgent activity in the capital city, Kabul. Officials noted, however, that the Soviets maintain the equivalent of two divisions -- roughly 20,000 men -- in and around the capital. eTrouble continues to erupt in cities and countryside areas where such overwhelming force is absent, the sources said.