Nearly two months after invading Afghanistan, Soviet military forces are bogged down in growing conflict with rebellious Afghan army units as well as increasingly well armed guerrilla bands, according to U.S. official reports from the area.

As portrayed by American analysts, the Russians have had to contend with just about every kind of setback possible, political as well as military, in the initial phase of the Afghanistan venture. Yet the same officials expressed no doubt that in the end the Russians will prevail over all the resistance.

The Soviet high command, described by the U.S. sources as dissatisfied with the campaign to date, is said to have replaced some of the senior Russian field commanders. Important Afghan leaders also have been removed, including the province chief, police chief and army division commander of troubled Jalalabad, situated between Kabul, the capital, and the porous Pakistan border.

In view of the deepening struggle, which is estimated to have cost the Russian military about 600 dead and 2,400 wounded in the past six weeks, the Soviets must decide whether they will increase substantially their force of 90,000 to 100,000 troops, the sources said. There is no sign at present of the kind of large-scale mobilization within Russia that would presage a major escalation, according to officials here.

Analysts with access to the full range of reports from around the world appear now to be discounting intelligence reports two weeks ago of unusual troop movements and mobilization in the Transcaucasian region of the Soviet Union near its border with Iran. These activities, which caused grave concern among high officials, are now described as a "field training exercise" on a normal schedule.

Given the growing difficulty of taming Afghanistan, it appears unlikely that the embattled Russian expeditionary force poses much of a threat in the short run to either of Afghanistan's neighbors, Iran or Pakistan.

It also seems less likely than even a week ago, as viewed by officials, that the Russians will be able to afford even a "cosmetic" withdrawal of forces from Afghanistan as a bid to reverse the tide of international concern about their action. Soviet officials have hinted at such a limited pullout in recent weeks.

Despite the dim prospects for a Soviet withdrawal of any kind, State Department sources said the United States has begun preliminary talks with several countries about setting up an international peacekeeping force to replace the Russians in Afghanistan. President Carter mentioned such an idea in his news conference Wednesday.

The Russian troubles in the harsh winter climate of Afghanistan, according to analysts here, include:

A crumbling and rebellious Afghan army, shrunken from about 70,000 men before the Soviet invasion to an estimated 40,000 or less. Far from the partner the Russians hoped for, the disintegrating Afghan military is seen here as an increasingly difficult Soviet foe.

Elements of the Afghanistan division centered near Nahrin in a mountainous area north of Kabul are reported to pose the most serious problems, waging a series of pitched battles with the Russians, from early January to early this week. The Russians are reported to have employed heavy air strikes, helicopter, gunships and ground forces in attempts to vanquish rebellious troops.

Increasingly active guerrilla forces, mostly tribal, which are said to have received an unexpected windfall of small arms and even heavy weapons from defecting Afghan regulars.

Official spokesmen at the White House, State Department and Central Intelligence Agency refused to comment on a Washington Post report that the United States has been covertly suppying Soviet-made small arms and antitank weapons to the Afghanistan insurgents since the Russian invasion. But there was every indication that, whatever the source, the guerrillas are increasingly well armed.

On the political scene, the continuing ineffectiveness of Afghan President Barbrak Karmal, who was installed after the previous communist leader, Hafizollah Amin, was killed at the start of the Soviet invasion Dec. 27.

U.S. analysts said information from the field has substantiated press reports of a new shootout involving some of the Afghan political leadership at the People's Palace in Kabul early in February. The Russians are believed to be still looking for a new leader to replace Karmal, who is rarely seen in public.