The Soviet Union has signaled the start of a major spring offensive in Afghanistan with attacks this week against Afghan rebel concentrations in the southwest corner of the country that may have strayed over into Iran, U.S. diplomatic and intelligence sources here report.

Analysts said the unusually large Soviet sweeps in the province of Farah--on Afghanistan's western border with Iran--were aimed at cleaning out hidden pockets of rebels who range across the flat desert land there.

A Soviet incursion into Iran would be its first in that country. Pakistan, another Afghan neighbor, claims more than 400 violations of its border by Soviet and Afghan forces during the past 28 months.

More strong sweeps are expected during the next two months with the Soviets using Mig fighters to strafe, bomb and fire rockets at suspected rebel concentrations before moving in troops by helicopter, intelligence sources said.

According to reports filtering back from Afghanistan, the Soviets began using tactical air power in a few winter assaults and found that it gave them a tremendous advantage against the rebels, who often were caught without any cover in the bleak Afghan countryside. It also took a greater toll on civilians, the reports said.

Analysts think the perfect time is now for the Soviets to begin new offensives as the mountain snows are melting but the spring vegetation has not yet come up to provide hiding places for the rebel mujaheddin--or freedom fighters. Furthermore, the MI24 helicopter gunships, which are the Soviets' best offensive weapon, work best in the spring or fall. They do not fly as well or as far in the hot, dry weather of an Afghan summer, according to military analysts.

"If you are going to put troops on the ground, you had better start now," said one intelligence officer who keeps a close watch on the fighting in Afghanistan.

The start of the spring offensive follows the annual winter lull caused by Afghanistan's extreme cold and heavy snows. The lull first was broken during the past six weeks by Soviet offensives in the provinces of Parwan and Wardak just west of the capital city of Kabul--where the Red Army scored real victories over the rebels--and in Kandahar, a major southern city that had been largely under insurgent control until the Soviets staged a brutal, block-by-block, World War II-style assault to take over.

"Kandahar," said an intelligence specialist, "had become a major embarrassment to the Soviets."

Information on the extent of the Soviet winter moves remains sketchy. But even rebel leaders in Pakistan's Khyber Pass city of Peshawar acknowledge large-scale losses to the Soviets, especially in Parwan Province. One report that reached New Delhi late last month placed the number of rebel fighters killed in the thousands--a rare admission for the mujaheddin--and said that among the dead where some key local leaders.

Nonetheless, the rebel activity has continued, and the Soviets control little more of Texas-sized Afganistan now than when they invaded with 85,000 troops in December 1979. Moscow has put more troops into Afganistan, bringing the total to around 100,000, but that is considered to be about one-quarter the number it would take to pacify the spunky rebel fighters.

As of now, the Soviets control a few major highways--although they, too, are open to ambush--and parts of some major cities.

The rebels, on the other hand, control much of the countryside, including a valley just a short car ride from Kabul. They are able to stage terrorist assassinations and kidnapings in the capital city, including the capture of a Soviet technical adviser who still is being held prisoner, and have established simple, government-style organizations in at least two regions, Hazarajat in central Afghanistan and the Panjshir Valley about 50 miles northeast of Kabul.

"The Russian invaders are generally able to keep their lines of communications open, to assure a continued flow of natural gas from Afghanistan to the Soviet Union and to bring overwhelming force to bear whenever a particular tactical situation warrants it," according to an article in the current issue of The Middle East Journal written by a man identified only as "a nearby observer" with "considerable experience in Afghanistan."

The flow of natural gas seems to pay in large measure for the Soviet occupation, as it is sold by Kabul to the Soviets at about 60 percent of the price at which the Soviets sell their natural gas to Western Europe.

The Soviet invaders appear to be trying to grind down the rebels while building up a trusted cadre of Moscow sympathizers among Afghan youth.