Israeli planes bombed a PLO guerrilla base near Sidon early Monday, killing all 12 of the fighters manning it, police and the PLO said. It was the highest death toll in 21 such attacks this year.

The police report said the jets destroyed a one-story base belonging to Yasser Arafat's Fatah, the Palestine Liberation Organization's largest faction.The base was in a lemon grove 71/2 miles southeast of Sidon, a southern port city that is the Palestinians' main stronghold in Lebanon.

In Jerusalem, the Israeli military command said its planes returned safely to base after hitting "a launching pad for attacks on Israel."

The air raid came a day after Soviet-designed Katyusha rockets landed in Israel's self-designated security zone and across the border in northern Israel. The rockets caused no damage or injuries.

Police and the PLO confirmed in separate statements that all the guerrillas were killed.

"There were no survivors. Those dug out alive died in ambulances before reaching hospital," a police spokesman said. He could not be named in line with standing rules.

Police say 28 people were killed and 82 wounded in the 20 previous Israel raids into Lebanon this year.

A PLO military communique issued in Tunis and sent to The Associated Press said the raid was part of a "massive military operation" Israel was planning in southern Lebanon.

It said Israel pushed troop reinforcements "estimated at two brigades" into its self-designated security zone in southern Lebanon three days ago, but gave no other details.

The reported buildup could not be independently confirmed.

Fatah, the largest and oldest of nine groups under the PLO, marks the 26th anniversary of its foundation on Tuesday.