Soviet-led forces and Afghan Moslem guerrillas are fighting increasingly fierce battles in eastern Afghanistan for control of important rebel supply routes from neighboring Pakistan.

Afghan resistance leaders based here and independent western observers said this weekend that fighting in Paktia and Nangarhar provinces, which border Pakistan, has intensified and moved closer to Pakistan during the past two weeks.

They said that admissions of wounded Moslem mujaheddin fighters to the Red Cross hospital here -- considered an indicator of the degree of fighting across the border -- are at their highest levels ever.

Guerrilla leaders and western diplomats in Islamabad have reported that helicopters and convoys of Soviet trucks have been ferrying troops and weapons into the border region during the past two weeks. Rebel spokesmen here said this weekend that Soviet warplanes and artillery have heavily bombarded resistance forces besieging the town of Khost in Paktia Province, about 25 miles from the Pakistani border.

"The Russians are trying to seal the border in this area to close off our routes into Afghanistan," said Isak Gailani, a leader of the National Islamic Front of Afghanistan.

Various groups said that Soviet and Afghan government forces tried and failed last week to push toward Khost from the west and that they are now attacking from the north, much closer to the so-called "parrot's beak" of Pakistani territory that serves as a major sanctuary for the guerrillas. A spokesman for the Hezb-i-Islami group led by Younis Khales, one of the main guerrilla factions active in Paktia Province, said the fighting is now centered near the town of Jaji, less than 10 miles from the Pakistani border.

At the Kacha Gari refugee camp here, Afghan men said resistance fighters were leaving the camps in large numbers to join the fighting across the border.

The mountainous terrain on the Afghan side of the "parrot's beak" is laced with mujaheddin supply routes that are the guerrillas' most direct links with Kabul, the Afghan capital.

During the recent fighting, both Pakistani and rebel sources have reported numerous bombardments of Pakistani villages by the Afghan government's artillery and air forces. Last Thursday, the Pakistani government formally protested to Kabul over the shelling of a village in the Kurram agency, or district, which forms the "parrot's beak."

Guerrilla leaders and western diplomats said incursions by Afghan government and Soviet forces into Pakistan normally increase just before each round of U.N.-sponsored talks on the Afghan problem, such as those held last week in Geneva.

"Moscow and Kabul seem to use these incursions as a tactic to pressure Pakistan toward making concessions" in the talks, a western diplomat in Islamabad said last week.

Three guerrilla groups cited reports from their commanders in four eastern Afghan provinces that a type of Soviet helicopter heretofore unseen in the Afghan war has been deployed in antiguerrilla operations during the past 20 days. Spokesmen said the helicopters, armored on the underside, are smaller and faster than the Mi24 "Hind" helicopters that have been the mainstay of Soviet airborne operations in Afghanistan.

"These helicopters are armed with guns and rockets and come in very quickly, firing on our dhashakas," or heavy machine guns, Isak Gailani said. "They also have a hook or claw which they lower down on the abandoned guns and pull them up out of their emplacements."

Spokesmen for the Younis Khales faction and the Jamiat-i-Islami group said today that they too had received reports that the new helicopters had taken rebel guns from their positions.

"Our dhashakas cannot harm them," said Jamiat spokesman Rasul Tarshi.

"Our commander in Logar Province [south of Kabul] sent a report saying they could only shoot one down with a grenade when it came low to take a dhashaka," Tarshy said. "After it crashed, they found many other dhashakas, which the helicopter had taken, in the wreckage."

The intensity and proximity of the fighting has dramatically increased the flow of injured Mujaheddin to the hospital operated here by the International Committee of the Red Cross. The 100-bed hospital had 160 inpatients during the weekend, and the staff hastily erected tents on adjacent land to shelter the wounded.

Michel Mordasini, the Swiss director of the hospital, said the organization was flying in more tents and an extra surgical team to treat the injured.

"The last months have been very busy for us, with about 200 admissions each month -- six or seven per day," he said yesterday. "But yesterday we had 15 admissions, and we've had nine by noon today . . . . Our surgical teams are working 12 to 15 hours each day."

Mordasini said that because the fighting is near the Pakistani border, the hospital is receiving guerrillas whose wounds are more severe than usual.

"Usually, men with head or abdomen wounds die before they reach us," he said.

Not all the wounded were fighters. In the hospital's intensive care ward, which spilled out onto an outdoor terrace, a child under 2 lay in traction, the lower half of his body shattered by shrapnel.