Afghan insurgents asserted today that they are holding their own in pitched fighting with a fully equipped Soviet invasion force of 40,000 men. The rebels report new defections from the Afghan Army to their side.

Other reports said the Soviets are further consolidating positions in the countryside and are now in control of all major cities and towns.

A spokesman for the Islamic Party of Afghanistan, which claims to be the largest rebel group fighting the communist government in Kabul, said insurgents battled Soviet forces in five provinces while large units of the Afghan Army were defecting with their equipment.

The rebel claims were sketchy and not immediately verifiable. Moslem factions fighting the Soviet-supported central government are disorganized, with the guerrillas often having fought each other in the past.

"The insurgents have been characterized by factionalsim," said a diplomat. "They don't have any rallying figure.There's no Ho Chi Minh."

Diplomatic sources said the onslaught of Soviet troops appeared to demoralize the guerrillas but could yet serve to unify them and increase their chances of acquiring outside help.

A rebel spokesman in this Pakistani provincial capital, about 60 miles from the Afghan border, said today that one Moslem group inside the country had shot down six Soviet aircraft and knocked out five tanks.

While diplomatic sources in Kabul, the Afghan capital, have disputed recent guerrilla accounts of the extent and intensity of the fighting, those sources have also affirmed that some serious fighting continues in the north and south.

More sporadic nighttime sniping was reported in Kabul, and the diplomatic sources said the Army desertions that began with the Soviet-engineered coup on Dec. 27 were continuing.

The rebel spokesman here said two Afghan military units in the Urgun district of Paktia Province defected with 40 tanks and 32 trucks loaded with weapons and ammunition. That province in southeastern Afghanistan has been the scene of some of the stiffest resistance to the new government.

The spokesman, Mangal Hussain, added that rebel forces loyal to the Islamic Party shot down two Soviet aircraft in that area and downed four others in the Kishin district of Badakhshan Province in the northeast. Diplomatic sources in Kabul have confirmed reports of fighting in that province in recent days.

The black-bearded Hussain, wearing an Afghan robe, said the fighting has been going on for three days and the rebels have killed "a great number" of Soviet troops. He claimed that rebel forces destroyed four Soviet tanks in Takhar Province and one near the town of Jalalabad, east of Kabul. tWhile he reported "heavy and bloody fighting" around Jalalabad between Islamic Party rebels and Soviet troops last night and this morning, truck drivers who passed through the town today said they saw no evidence of any battles.

Hussain said the party's information on the developments in Afghanistan comes from Afghan refugees crossing the border into Pakistan and from radio transmissions that he said have been sent from rebel strongholds inside Afghanistan.

The spokesman, a former teacher in Kabul, indicated that no further progress has been made toward uniting the various factions that began waging a disorganized guerrilla war following the first Soviet-backed coup in April 1978 that installed an initial pro-Moscow communist government.

Hussain said there was no meeting today of the Islamic Party and three other Moslem rebel groups that gathered here Thursday and Friday to discuss pooling their forces to face the Soviet invasion.

"Much of the insurgency has been spontaneous and local," a diplomat said, and some groups appear to have been engaged mainly in banditry.

A report by the previous Afghan government identified 64 semi-independent rebel groups, the diplomat said. The disunity poses a problem for any outside government or organization seeking to help the rebels.

"Who do you help?" the source said. "Who do you deal with?"

For his part, Hussain complained bitterly that no aid has come from Moslem countries.

"We're fighting a holy war in Afghanistan," he said. "It's the responsibility of every Moslem all over the world to support the Moslems of Afghanistan or take part in the jihad against the Russian troops."

Hussain had particularly harsh words for Afghanistan's Islamic neighbor to the west, Iran. "Unfortunately, we haven't received a single penny, a single bullet from the Iranian government," he said. All he had to show from Iran were a couple of memorial posters on the office wall behind him of two ayatollahs who died last year.

Hussain said individual Moslems in various countries and Afghans working abroad have been supporting the Islamic party financially. Most of the rebels' weapons, he said, are Soviet arms captured from Afghan Army or Soviet forces or brought over by defecting Afghan soldiers. Some weapons have been procured from private Pakistani arms manufacturers in this unruly North West Frontier Province.

Hussain said the guerrillas' arsenal before the latest reported Afghan Army defections included Soviet artillery pieces, heavy machine guns, armored personnel carriers, antiaircraft guns, tanks and grenade launchers. A few helicopters were also captured, he said, although the rebels' ability to fly and maintain them appears dubious.

As proof of the acquisitions he showed photographs of turbaned guerrillas posing with various captured weapons. Other snapshots showed the bodies of Afghan communist provincial officials slain by the rebels. One photo showed what was said to be an unexploded napalm bomb dropped on Afghan rebels.

Asked if his party wanted to set up an Islamic republic like Iran's under Ayatollah Ruhollah Khomeini, Hussain said, "Khomeini has not succeeded in applying Islamic principles properly."

He added, "If we can seize power in Afghanistan, we will establish a real Islamic state."