The Syrian air force on Sunday reportedly bombed the Yarmouk refugee camp outside Damascus, the largest Palestinian refugee camp in Syria, killing dozens, opposition sources reported.

Dozens died and dozens more were injured when planes fired rockets at a mosque in an area where Syrian rebels have been trying to advance into the capital, opposition activists from the Syrian Observatory for Human Rights told Reuters.

Videos posted to YouTube appeared to show the bodies of the victims sprawled outside the entrance of a mosque in the refugee camp, home to over 150,000.

Palestinians are divided over the crisis in Syria. When the unrest began in March 2011, the half-million-strong community tried to stay on the sidelines. But in recent months, many Palestinians started supporting the uprising, although most insisted the opposition to the regime should be peaceful.

On Sunday, for the first time since the start of the uprising, the PLO condemned the Assad regime for its attack on civilians.

“We call on the warring sides in Syria to spare the Palestinian people and their camps in Syria,” Palestinian Authority President Mahmoud Abbas said in a statement carried by official Palestinian news agency WAFA, adding that bombing of the refugee camps “must be stopped immediately.”

“We also call on the international community to take immediate action to provide protection to our people in Syria,” Abbas said, adding that the Palestinians were “not involved” in the conflict

“We hold Bashar Assad and his regime responsible for this crime in Yarmouk camp, which clearly reveals that this regime knows no limits in its approach to criminal murder and destruction,” Senior PLO official Yasser Abed Rabbo told Reuters.

“In the Yarmouk refugee camp massacre and everywhere in Syria, the international community must put an end to a system of murder and terrorism in Syria before they burn the whole region,” Rabbo said. “We are following up with our people in Syria about the conditions on the ground and we will take all measures that enable us to protect our people at all levels.”

Hamas also condemned the attacks, calling them a “crime.”

Several Palestinian groups with longstanding ties to the regime are fighting on the government side, including the Popular Front for the Liberation of Palestine-General Command. The group is led by Ahmed Jibril, a strong ally of Assad.

Jibril, according to rebel sources, fled Yarmouk on Sunday with his son bound for the Mediterranean city of Tartous, a stronghold of Assad’s minority Alawite community. Opposition sources later reported that the PFLP General Command had joined the rebels in their fight against the Assad regime. It was not clear under what circumstances or why Jibril fled to Tartous.

Abdul-Rahman said the fighting, concentrated on the main street in the heart of the camp about six kilometers (four miles) from downtown Damascus was still in progress late Sunday afternoon.

The Observatory relies on reports from activists on the ground, which cannot be independently verified.

A resident of Yarmouk told the Associated Press that the clashes between rebels and PFLP-GC gunmen flared up on Friday when rebels tried to take over the PFLP-GC’s headquarters in Yarmouk. The resident spoke condition of anonymity for fear of reprisals.