BEIRUT, JULY 3 -- Israeli jets attacked a militia base in the Syrian-controlled Bekaa Valley today, wounding 13 persons, security sources and witnesses said.

The raid, Israel's 18th against Lebanon this year, targeted a base of the National Syrian Social Party, a leftist Lebanese group closely affiliated with Syria.

A villa owned by Information Minister Joseph Skaff, and now housing squatter families of the National Syrians, was badly damaged in the attack. Two other buildings and five cars were wrecked.

A party official visiting the scene said the raid appeared to be in retaliation for a National Syrian guerrilla attack last month on Israeli and Israeli-backed militia forces inside Israel's self-declared security zone in South Lebanon.

The raid also may have been linked to an earlier bus bombing in Israel and a foiled infiltration attempt by Palestinian guerrillas.

In Israel, the driver of an Israeli bus was injured today when a bomb exploded on the bus near Ashkelon, south of Tel Aviv, police said.

An anonymous caller told an international news agency that Force 17 of the mainstream Fatah organization was responsible for the bus attack. Damascus radio reports said rival Fatah dissidents were responsible.

Military sources in Tel Aviv said the Israeli-backed South Lebanese Army killed two Palestinian guerrillas of the Syrian-based Saiqa movement as they were attempting to infiltrate into northern Israel by sea yesterday, a few hours before the Israeli bomb attack.

The sources were quoted as saying a South Lebanese Army patrol spotted the guerrillas on a dinghy heading for Israel.