Hundreds of residents of a main Palestinian camp in Lebanon fled on Monday as fighting between mainstream faction Fatah and radical Islamists raged for a third day, residents and security sources said.

The fighting wrecked a ceasefire agreed on Sunday between rival Palestinian groups that included representatives of pro-Iranian Hezbollah group and its ally Shi’ite Amal movement that hold sway in southern Lebanon.

At least 11 people have been kllled and 40 wounded in the clashes that broke out in Ain el-Hilweh camp, near the southern coastal city of Sidon, over the weekend, security and Palestinian sources said.

On Monday, the warring groups fired rocket-propelled grenades at each other in the crowded alleys of the camp. Both sides blamed the other for the breach in the ceasefire.

Palestinian sources say the latest round of fighting on Saturday was triggered by a security clamp-down by Fatah, the main faction in the camp against hardline Islamists who have a foothold there.

A death of a militant led to attacks on Fatah outposts.

Fighting intensified on Sunday after a Fatah commander was killed in an ambush set up by militants and where three of his companions later died from wounds in what seen as a reprisal for the Islamist militant’s death the day before.

More than 2,000 people were forced to flee, seeking safety, said Dorothee Klaus, head of UNRWA, the UN agency responsible for the welfare of Palestinian refugees, which runs basic services in the camp with over 50,000 inhabitants.

It had opened schools to accommodate displaced families, Klaus said.

Shrapnel injured several Lebanese soldiers outside the walled camp were the army control access for people in checkpoints outside.

Some 400,000 refugees live in Lebanon’s 12 Palestinian camps, which date back to the 1948 war between Israel and its Arab neighbors. The camps mainly lie outside the jurisdiction of Lebanese security services.