An Israeli gunboat sank a dinghy off the Lebanese coast Saturday, killing five armed guerrillas who were apparently en route to attack an Israeli target, according to radio reports and military sources.

It was not known if there were any survivors from the dinghy, Israeli radio reported. It said there were no Israeli casualties.The Israeli military had no official statement on the incident late Saturday, but military sources, who spoke on condition of anonymity, confirmed the sinking and said it was under investigation.

Reports on Israeli radio and broadcasts in Beirut said the guerrillas were planning an attack in Israel.

The Moslem Voice of the Nation and the Christian Voice of Lebanon said the dinghy sank off Saadiyat, about 12 miles south of Beirut.

They did not identify the guerrillas killed or provide any other details.

The reported clash came four days after Israeli troops and allied militiamen of the South Lebanon Army ambushed a Palestinian guerrilla squad in southeast Lebanon.

Police and the Israeli command said four guerrillas and an Israeli officer were killed in the eight-hour firefight near the village of Ain Atta, in the foothills of Mount Hermon.

The Libyan-backed Palestine Popular Struggle Front, a small radical group of Yasser Arafat's Palestine Liberation Organization, identified the guerrillas in that battle as members of its force.

An abortive attack on an Israeli beach by seaborne guerrillas in May prompted the United States to cut off its 18-month dialogue with the PLO. That attempt was claimed by an Iraqi-backed group, the Palestine Liberation Front.