Security officials said a car bomb Friday killed a leader of Palestinian Islamic Jihad — an Iran-backed militant group that persisted in attacking Israel while other major factions adhered to a cease-fire.

His brother was also killed in the explosion, which the group blamed on Israeli intelligence, and it vowed to retaliate.

"With God's permission, the response will be sharp," said Abu Imad Rifai, Islamic Jihad's representative in Lebanon.

Mahmoud Majzoub, a member of the group's policy-making Shura Council body and its leader in Sidon, 24 miles south of Beirut, was walking with his brother, Nidal, near the central square of this coastal city when a parked car was detonated by remote control, security officials said.

Mahmoud Majzoub survived the blast but died during surgery, while Nidal Majzoub was killed instantly, said the security officials, who spoke on condition of anonymity because they were not authorized to release the information.

Islamic Jihad's representative in Lebanon said Nidal Majzoub also was a member of the group, which has continued to launch attacks on Israel since a February 2005 truce that the main militant group Hamas, which swept Palestinian legislative elections, has respected.

The explosion shattered the windows of several nearby apartment buildings. Lebanese police and soldiers sealed off the area as dogs searched for additional bombs.

"I rushed to the balcony after hearing the blast only to see black smoke billowing from a car," said Omran Kaddoura, 16. After running down six flights of stairs to the street, he "found the two bloodied men lying on the ground in front of the car."

Before Mahmoud died, Islamic Jihad's representative in Lebanon, Abu Imad Rifai, blamed Israeli intelligence for the attack.

"This morning the Israeli Mossad detonated an explosive charge. ... This is the second time Mahmoud is targeted, and I believe it comes in the framework of Israeli escalation against the Palestinian people, particularly Islamic Jihad," Rifai told The Associated Press.

Mahmoud, 41, his wife and infant son were wounded in an explosion in 1998 in Sidon when their booby-trapped car detonated. Islamic Jihad blamed Israel for the blast, which also injured a passer-by.

Rifai confirmed that Nidal, 39, was a member of the group.

In Israel, military officials said they had heard about the bombing through media reports, but had no additional information.

Rifai rejected the possibility that other Palestinian or Lebanese groups were behind the attack.

"No one has an interest in assassinating him except the Israeli Mossad," he said.

Palestinians have blamed the assassinations of several militant leaders on Israel. But some of the killings of leading members of Palestinian groups were the result of intra-Palestinian feuds.

Islamic Jihad says its attacks on Israel are a legitimate response to a crackdown on its members.

The virulently anti-Israel group backed by Iran and Syria claimed responsibility for an attack that killed 11 and wounded dozens in April in Tel Aviv, the deadliest Palestinian attack in 20 months.

Islamic Jihad is led by Ramadan Shallah, a Palestinian from Gaza who now lives in exile in Syria. It considers the 1979 Iranian Revolution to be the beginning of a new era for the Muslim world and wants to turn all of Israel, the West Bank and Gaza into an Islamic state. It rejects compromise with Israel.