PESHAWAR, PAKISTAN, OCT. 18 -- An Afghan resistance leader says Iran seized nine U.S.-made Stinger antiaircraft missiles in a skirmish with Afghan guerrillas in May.

The guerrilla chief's statement supported reports circulating in Washington that Iran had acquired Stingers. The reports arose after Iranian forces fired what appeared to be a Stinger at a U.S. helicopter in the Persian Gulf this month.

Yunis Khalis, head of one of the main Moslem organizations fighting Afghanistan's Marxist government and an estimated 115,000 Soviet troops, said yesterday that Iranian border guards seized ammunition and the heat-seeking missiles when his guerrillas strayed into Iranian territory in late May.

{Seven main Afghan guerrilla groups in Pakistan have elected Khalis first president of their alliance, according to sources within the group quoted by Reuter in Islamabad. Khalis was due to hold a news conference Monday to make the announcement.}

On the Stingers, Khalis said a five-truck convoy trying to cross a swollen river in western Pakistan's Farah Province was a few hundred yards into Iran when border guards tried to stop it.

The guerrillas mistook them for Afghan government troops and opened fire, killing one, Khalis said through an interpreter. When Iranian reinforcements arrived, the guerrillas tried to flee but only three trucks escaped, he said. The remaining two trucks, weapons and ammunition were taken to the Iranian border town of Zahedon.

Khalis said that when news of the incident reached Peshawar, the Iranian consulate assured him it would return the equipment, but later reneged.

In another development, Agence France-Presse reported from Moscow:

Afghan leader Najibullah acknowledged that his government's policy of national reconciliation was running into difficulties, with Army units facing problems on the ground from U.S.-backed rebels and the ruling Communist Party riven with factional infighting.

The Afghan leader told the opening session of a three-day Afghan Communist Party conference in a speech in Kabul, reported by the official Soviet Tass news agency, that "not all Army units" were able to face the enemy.

"Not all problems of the armed forces have been solved," he said, explaining that the situation since the national reconciliation had been proclaimed last January was "more complicated" than previously thought.

"The national reconciliation policy has not yet become irreversible," he said. Najibullah told the 677 delegates present that 1,600 villages had gone over to the side of the authorities in the last 10 months. The government now controlled "more than one-third" of inhabited villages, 45 towns and 214 districts and rural districts, he said.

The total number of towns and districts was not known, but Najibullah's statement appeared to indicate that the rest of the country was controlled by the rebels.

The Afghan leader held out a new olive-branch to the Pakistani-based opposition forces, offering them representational offices in Kabul on condition that they showed willingness to compromise. Future contacts with the foreign-based rebels needed to attract "moderate and neutral forces," he said, stressing that the ruling party did not want to retain a monopoly on power.