Christian militiamen in southern Lebanon today ambushed an unarmed United Nations peacekeeping force convoy, abducted three Irish soldiers and later exeucted two of them, dumping their bodies near a U.N. position, officials at the U.N. headquarters here said.

The murders were described by the U.N. officials as vengence killings for the deaths of two militiamen during a clash with the Irish U.N. troops on Saturday near the village of At Tri. After the battle, the militia had demanded either $13,000 in reparations or the bodies of any two members of the Irish battalion of the U.N. peacekeeping force.

[The U.N. Security Council, after meeting in closed-door session, strongly condemned all those responsible for "this unprecedented, barbaric act." It also announced it would take "determined action" to enable the peacekeeping force to take immediate control of the entire area near the border.]

[A spokesman for the Irish Embassy in Beirut was quoted by Reuter as saying that one of the slain U.N. soldiers had been assaulted sexually before being shot in the back.]

A U.N. spokesman here said the peacekeeping force's southern Lebanon headquarters at Naquora had been assured earlier in the day by the Israeli Army and by Maj. Saad Haddad, commander of the Iraeli-supported Christian militia, that the abducted Irish soldiers were safe in militia custody.

A third Irish soldier, Pvt. John O'Mahoney, was caught in today's ambush near the village of Bint Jbail, in the narrow enclave controlled by Haddad's forces. He was shot in the leg and stomach. He was taken to the Irish battalion headquarters at the village of Tibnin and then evacuated by helicopter to a U.N. hospital at Naquora.

Of the two dead Irish soldiers, Hugo Rocha, spokesman for the United Nations force in Lebanon (UNIFIL) said, "These two innocent drivers were the first unarmed peacekeeping soldiers to be murdered in cold blood in southern Lebanon."

Tensions between Haddad's militia -- which is paid, equipped and advised by the Israeli Army -- and the Irish battalion have been steadily rising in the past three weeks following a series of clashes in the area of southern Lebanon assigned to the Irish troops.

U.N. officials said three U.N. vehicles, driven by unarmed soldiers, were headed toward the observation post of Maroun al Ras on the Israel-Lebanon border when they were ambushed at Bint Jbail, a Haddad-controlled village.

In the group were an American officer, Maj. Harry Klein, and French Capt. Patrick Vincent of the United Nations Truce Supervisory Organization, and a reporter and photographer from the Beirut bureau of the Associated Press. The four were detained briefly by the militiamen, but later released unharmed.

Steve K. Hindy, the Associated Press reporter in the group that was seized, gave the following account of the incident:

The half dozen gunmen jumped out from behind the stone wall on either side of the road and ordered us out of our vehicles. They were led by a man about 30 years old who shouted that he wanted to avenge the death of his brother.

After taking the guns of the U.N. soldiers, they pushed four of them into a Peugot 404 and drove us over a rutted mud schoolyard to a deserted school. When AP photographer Zaven Vartan told them he was a Lebanese-Armenian, they told him to stay upstairs.

The rest of us were rushed, at gunpoint, down a stairway into a filthy toilet with shattered fixtures and glass on the tile floor.

A young gunman wearing a red beret asked our nationalities and we told him. "American, good. French, good," he said.

Two gunmen prodded the Irishmen down a long corridor off the bathroom and disappeared. We paced for about 10 minutes before the shooting echoed down the barren corridors of the building.

Two of the Irishmen bolted and ran through a passage into the arms of Haddad gunmen waiting outside. O'Mahoney staggered out of the door. The gunmen herded him back into the room.

There was more shooting, inside and outside the building.

We were released near Bint Jbail about an hour after being captured and took the wounded O'Mahoney to Tibnin in a taxi. The leader of the gunmen took the two other Irishmen away in his car, along with the AP photographer's cameras, and the bodies of the two men were later dumped near a U.N. position.

Reuter reported that militia commander Haddad denied his men were involved, saying the Irish soldiers were taken by members of a Moslem family whose son was shot in the clash last weekend between the Irish U.N. troops and the militia.