A drone strike in the southern Lebanese city of Sidon has killed an official from Fatah, the movement led by Palestinian leader Mahmoud Abbas.
State media and Palestinian officials in Lebanon said Israel had carried out the attack.
"The Israeli strike in Sidon killed [Fatah] group official Khalil [al-]Makdah," said Fathi Abu al-Aradat, a senior member of the political party.
There have been no previous attacks on Fatah in more than 10 months of cross-border clashes between Israel and Lebanon's Hezbollah movement.
A Lebanese security source confirmed the report to AFP, saying the strike hit his car.
Lebanon's official National News Agency said Makdah was killed "in a drone strike on his car."
Mounir al-Makdah, who heads the Lebanese branch of Fatah's armed wing, the Al-Aqsa Martyrs' Brigades, confirmed that his brother Khalil had been killed.
He told the broadcaster Al-Mayadeen that his brother had been a commander in the brigade.
There was no immediate comment on the attack from the Israeli military.
The Islamist militant group Hamas and Fatah have been bitter rivals since Hamas fighters ejected Fatah from the Gaza Strip after clashes that followed Hamas's resounding 2006 election victory there.
Fatah still controls the Palestinian Authority, which has partial administrative control in the Israeli-occupied West Bank.
Israeli confirms it targeted Fatah official
The Israeli military says it targeted a commander in the armed wing of the Palestinian Fatah movement, describing him as having worked for the Iranian Revolutionary Guards.
Khalil al-Maqdah was killed in a targeted air strike in Lebanon on Wednesday.
"Earlier today... an air force aircraft targeted Khalil al-Maqdah in the Sidon area of southern Lebanon," the military said in a statement.
It claimed that Maqdah and his brother worked for Iran in "directing attacks and transferring funds and weapons to terrorist infrastructure" in the occupied West Bank.
The "assassination of a Fatah official is further proof that Israel wants to ignite a full-scale war in the region," Tawfiq Tirawy, a member of Fatah's central committee, told the AFP news agency.