The fighting between Fatah and Islamists in the camp, which erupted overnight, killed a Fatah military leader and four of his colleagues, the secularist movement said.
A Palestinian source inside the camp, speaking on condition of anonymity, said an “Islamist from the al-Shabab al-Muslim group” was also killed and six others including the group’s leader were wounded.
An AFP journalist said clashes at Ain al-Helweh, the largest of the 12 Palestinian refugee camps in Lebanon, were still ongoing on Sunday afternoon.
Fatah in a statement confirmed the death of commander Ashraf al-Armouchi and four of his “comrades” during a “heinous operation”.
The statement denounced an “abominable and cowardly crime” aimed at undermining the “security and stability” of the Palestinian camps in Lebanon.
A Lebanese soldier was also wounded, hit by shrapnel from “a mortar shell that fell in one of the military posts”, the army said on Twitter, which is being rebranded as X.
His condition was reported as stable.
Fighting between rival groups is common in Ain al-Helweh, which is home to more than 54,000 registered Palestinian refugees who have been joined in recent years by thousands of Palestinians fleeing the conflict in Syria.
By long-standing convention, the Lebanese army does not enter Palestinian refugee camps in the country, leaving the factions themselves to handle security.
The camp has gained notoriety as a refuge for extremists and fugitives.
More than 450,000 Palestinians in Lebanon are registered with UNRWA, the UN agency for Palestinian refugees.
Most live in one of the 12 official refugee camps, often in squalid conditions, and face a variety of legal restrictions, including on employment.