Soviet ground forces, assisted by airborne troops, have pushed well over halfway through Afghanistan's strategic Panjshir Valley, Western diplomatic sources confirmed this week.
They conceded, however, that they were receiving highly contradictory accounts on whether additional airborne reinforcements had arrived and secured the crucial Anjuman Pass. Reports from Afghan resistance sources in Peshawar, Pakistan, over the weekend claimed the Soviets had blocked the northern route into the valley by securing the northeastern pass.
According to the Western sources, the Soviets, in their largest offensive in the 41/2-year-old war, appear intent on encircling the valley. It is the stronghold of one of Afghanistan's most renowned guerrilla fighters, Ahmed Shah Massoud - the only resistance leader with whom the Soviets have ever negotiated. Last January, the two sides entered into a year-long truce.
The scope and intensity of the Soviets' Panjshir offensive and the Soviets' dogged effort to capture Mr. Massoud, add credence to reports of a decided hardening in the Kremlin's Afghanistan line since the accession of Konstantin Chernenko in February of this year.
Yuri Andropov had hinted he was interested in a negotiated solution to what some in the Kremlin have conceded was a vexing foreign war.
All indications from the Kremlin now, however, are that Mr. Chernenko is not.
''In short,'' said an Eastern diplomatic official, ''Andropov gave high priority to the United Nations-sponsored talks. We've seen during the April visit of (the UN mediator, Diego) Cordovez, that Mr. Chernenko is not at all interested in a Soviet withdrawal . . . and he isn't because, despite the conventional wisdom that has coalesced the West, the new Soviet leadership does not believe that its Army is bogged down in Afghanistan. . . .''
''Time and resources are clearly on the Soviet side,'' he continued. ''For a very limited investment, they have made significant geopolitical gains.''
Mr. Massoud, a former engineering student - known to his followers as ''the Panjshir lion,'' and to the Soviet Red Army as ''the scarlet pimpernel,'' - is now safely out of the valley, according to Western accounts. He left on April 18 , three days before the beginning of the Soviets' newest drive.
Before he left, he instructed his 5,000 fighters not to engage the Soviets directly, but to break into pockets of resistance, sheltering in peripheral valleys and along the ridges of the precipitous Hindu Kush mountain range, which pose difficult physical obstacles for a Soviet pursuit.
According to one report, Mr. Massoud is meeting with other resistance leaders north of the valley and planning a series of attacks on the Salang Tunnel, a key supply and transport route between the Soviet Union and the Afghan capital, Kabul.
The new Kremlin thinking is said to center around the fact that the 110,000 to 120,000 Soviet troops shoring up the regime of Babrak Karmal can afford to sustain the 1,000 casualties that they have suffered each year. According to Soviet sources, only about 300 are actual combat deaths. The other deaths are attributed to hepatitis, local fevers, alcohol poisoning, and suicide.
Only a small percentage of the conscripts - as few as 15,000 to 20,000 according to one military attache - are reportedly exposed to the full rigors of war. The others are said to be involved in ''education and reorientation'' programs, building new airstrips and roads, protecting vital Afghan Army installations and transport and communications sites.
The Soviet Air Force has taken over and expanded military air bases at Kandahar, Shindand, Farah, Kabul, Bagram, Jalalabad, and Herat. Their present air and ground deployment thus gives them the capability for an expeditious thrust into the Indian Ocean, Iran, and the Gulf.
But the lush Panjshir Valley has been a longtime irritant to the Soviet regime, and both the stronghold and symbol of the Afghan mujahideen.
Thus three separate Soviet armored columns are pushing through the valley - 15,000 Soviet and 2,000 Afghan forces, along with 400 to 600 tanks. They are supported by unprecedented, high-altitude, saturation bombing raids. As many as 100 Soviet TU-16 Badger bombers and SU-24 Fencer fighters arrived in Afghanistan from the Soviet Union at the end of March. It is a scale and intensity of commitment never sanctioned by Mr. Andropov.
Casualties have been minimal, according to Western diplomatic reports, as virtually no civilians remain in the 70-mile-long valley. Contact between the advancing Soviet forces and the mujahideen has been limited to occasional hit-and-run raids by the resistance.
The Western officials quoted above have also discounted the veracity of Afghan resistance reports that the Soviets used chemical weapons as they have continued their advance, now into its third week.
''It's simply illogical,'' an official said, ''when the valley's been evacuated, and there are virtually no inhabitants left.''
According to an Eastern European source, the operation is taking longer than the Soviets had expected when they launched the offensive April 21. They are said, as during previous Panjshir campaigns, to be having difficulty in flushing the mujahideen out of their new deployments in side valleys and along the mountain ridge. Six earlier attempts to take the valley have failed.
The seeming Soviet reluctance to continue pursuing a carrot-and-stick approach in Afghanistan began to appear as early as the February funeral of Mr. Andropov.
Fearing that Pakistan's borders were increasingly vulnerable, and saddled with 3 million increasingly disruptive Afghan refugees, Pakistani President Muhammad Zia ul-Haq went to Moscow for the funeral. He was fully prepared to propose direct Pakistani-Soviet talks on Afghanistan to Mr. Chernenko, a move initiated earlier by the Islamabad government, but which the Kremlin had rebuffed.
There was no such opportunity. Despite two formal requests for an appointment , Mr. Chernenko pointedly ignored the Pakistani leader. Instead, Chernenko met with India's Indira Gandhi, Babrak Karmal, and a relatively unknown admiral from Bangladesh.