Washington — The ragtag Afghan guerrillas have made ''steady advances'' in organization and fighting ability over the past year, according to a new United States Senate staff report.
At the same time, the Soviet Union's initial plan to secure the Afghan countryside with military surrogates has foundered on the Soviet-backed Afghan Army's low morale and high desertion rate, the report says. As a result, it says , Moscow has ''intensified its efforts to brutalize the Afghan population into submission.''
The 57-page Senate Foreign Relations Committee staff study argues, however, that despite massive Soviet ground sweeps, heavy air strikes, and ''particularly brutal acts of reprisal,'' the Afghan guerrillas have shown an impressive ability to carry the war into the Soviet-held cities.
''The unexpected strength of the resistance has blunted initial fears that the Kremlin might seize Afghanistan as a base for further aggression'' in the direction of Pakistan and the Arabian Sea and the oil-producing Gulf, the report says.
The report was written by John B. Ritch III, a senior Foreign Relations Committee adviser on European and Soviet affairs. Mr. Ritch visited Pakistan
Jan.28-Feb. 8, during which time he met Pakistani President Zia ul-Haq in Islamabad and with resistance fighters in Peshawar, and then crossed into Afghanistan for several days with a group of mujahideen, or freedom fighters. Ritch is the first US government official known to have visited Afghan guerrillas in the field. He was allowed access to 14 Soviet prisoners being held by the guerrillas. He says there may be more than 200 of them.
Acting on authorization from Sens. Charles H. Percy (R) of Illinois and Claiborne Pell (D) of Rhode Island, Ritch visited Pakistan officially - and Afghanistan unofficially - to prepare a report on the war. Senator Percy chairs the Foreign Relations Committee. Mr. Pell is the committee's ranking Democrat.
In his report, Ritch describes the current state of the war as a ''standoff'' but challenges the common assumption that a Soviet-dominated Afghanistan is a fait accompli.
This assumption, he says, must be balanced against ''the extraordinary courage and passion'' of an Afghan resistance fired by a ''fanatic, religiously inspired zeal'' to drive out the Soviets.
''How Soviet policy may evolve in the face of such an implacable resistance - an enemy fully prepared to die in martyrdom - must be regarded as an open question,'' Rich said.
Ritch finds little hope for progress toward a settlement of the war through the United Nations-sponsored talks now under way in Geneva between Pakistan and the Soviet-backed Babrak Karmal regime unless Moscow accepts the concept of Afghanistan's return to the status of a neutral, independent country. Such a change in Soviet policy would be possible only as a result of increased military and political pressure, Ritch says.
He recommends that, in close coordination with the 42-nation Islamic Conference, the US encourage the unification of the resistance fighters and the creation of an Afghan government in exile. In Ritch's view, this would serve to increase the pressure on the Soviets and thus facilitate a political settlement.
''The establishment of an Afghan government in exile, validly representing the nation's noncommunist majority, would by no means ensure a change in Soviet policy,'' Ritch says. ''But it would provide a basis for a real negotiation on Afghanistan's future, in the event that increased military and political pressure were to incline Moscow to temper its aspirations. . . .''
For more than a year, the US Congress has focused its attention on a proposed resolution declaring that the US should provide the people of Afghanistan, if they so request, with material aid. But opposition to such a resolution, led by Republican Sen. Charles McC. Mathias Jr., of Maryland has centered on the possibility that such a resolution would mislead, and consequently disillusion, the mujahideen.
Mr. Mathias has also been concerned that the proposed resolution might compromise the delicate position of the Zia regime, bolster the Kremlin's propaganda efforts, and constitute an open-ended, expression of legislative.
Ritch recommends a revised, three-point resolution:
* American aid given to the resistance ''in close consultation and coordination with nations of the Islamic conference.''
* A US declaration of readiness to recognize an Afghan government in exile, if a representative entity can be formed.
* Immediate action by the US and its allies to accept and assimilate Soviet prisoners in Afghanistan who seek refuge.
According to Ritch, this resolution would place the US within a multilateral, Islamic-oriented effort, thus avoiding the danger of embarrassing Pakistan and assisting Soviet propagandists. He says it would also create a ''powerful stimulus'' to unification among the mujahideen while providing them with an immediate boost in morale.