A Soviet drive into Afghanistan's Kunar province brought a Russian regiment "within a stone's throw" of the Pakistani border yesterday, according to U.S. government analysts, and probably marks the start of a major spring offensive.

But the officials said there was no sign, nor any expectation here, that the Soviets would cross the rugged and ill-defined border into Pakistan. The few thousand Soviet troops that arrived in the border garrison at Barikout seemed to be part of the operation launched this week -- the biggest Soviet onslaught since the invasion of Afghanistan in December -- throughout the 120-mile-long province in northeast Afghanistan.

Officials said that, with the weather improving and following a shake-up in the Soviet military command, Moscow's forces appear to be better organized, increasing their use of firepower and doing it more effectively than in the previous two months.

These officials and State Department spokesmen said yesterday that there were signs that the Soviets were increasing their presence in Afghanistan and planning to stay permanently or for a long time.

The analysts said that two or three senior Soviet generals in charge of military construction had been in Afghanistan recently. State Department spokesmen said that the construction -- including living quarters and what appear to be military clubs as well as cable-laying and well-digging -- pointed to a long stay.

The analysts estimated that there were about 80,000 Soviet troops in Afghanistan, with 25,000 more along the borders, and that at least two divisions totaling 25,000 to 30,000 troops probably would enter the country soon. The analysts estimate that at the height of the spring offensive the Soviets may have 120,000 to 130,000 troops in Afghanistan.

The offensive is meant, in the analysts' view, to re-exert Soviet control over several key provinces.

However, these officials believe that the Soviets still will not have enough troops to occupy these regions permanently, and eventually will move back into sanctuaries in the most important areas for control of the country and along the borders.

Thus, the analysts think that while the Soviets can now more easily drive insurgent forces out of wide areas, if the insurgents avoid direct contact the Soviets will not be able to defeat them or prevent them from seeping back into the valleys.

Of all the heavy weapons -- including tanks and fighter bombers -- being used against insurgent strongholds, the analysts seemed to stress that the Soviet helicopter gunships were the most "awesome" weapons the rebels faced because the choppers could hit them in mountain hideaways.

These comments come amid rumors here that consideration is being given to trying to equip rebel forces with heat-seeking, antiaircraft missiles that can be fired by one soldier. These missiles are produced in several countries.

The U.S. analysts are sticking to their estimates of 400 to 600 Soviet casualties from all causes each week in Afghanistan, including about 100 killed. They say insurgent losses are several times those of the Soviets, but there are no reliable estimates.

The analysts also believe that, while the Soviets may have run into more problems in Afghanistan than they bargained for, their setbacks are nowhere near severe enough to make them turn back.

Responding to persistent reports from refugees that the Soviet troops were using poison gas, the analysts said it seemed clear that the Russians were using some gas, possible teargas or the type used in riot control, but that there was "no convincing evidence" at this point that any lethal gas was being used.