GAZA,(Reuters) – Fighting between Palestinian factions escalated across Gaza on Friday, killing at least 15 people as Hamas overran compounds used by President Mahmoud Abbas’s forces and two major universities were set ablaze.

Residents of the narrow coastal strip, home to 1.5 million Palestinians, hid in their homes instead of attending Friday prayers as the rivals fought running gun battles with rifles and rocket-propelled grenades from streets and rooftops. “Gaza is being burned down,” Arafat Abu Eyad said from his smoke-filled balcony overlooking smouldering buildings.

At the urging of Saudi Arabia’s King Abdullah, Abbas of Fatah and Hamas leader Khaled Meshaal agreed to hold talks in the holy Muslim city of Mecca, most likely on Tuesday, a senior Palestinian diplomat said.

Eight fighters loyal to Abbas, four Hamas gunmen, a woman and two children were killed on Friday in a second day of fierce clashes in the Gaza Strip that shattered a three-day-old ceasefire between Fatah and the ruling Islamist movement.

At least 21 Palestinians have been killed and more than 200 wounded in the last 24 hours of internal fighting.

The United States has convened a meeting of the Quartet of Middle East mediators in Washington on Friday, seeking to bolster support from the European Union, Russia and the United Nations for its embargo of the Hamas-led government and its attempt to revive stalled peace talks through Abbas, a moderate.

The United States has pledged $86 million to strengthen Abbas’s security forces. Hamas accuses Fatah of spearheading a U.S.-backed coup against its government.

Jamal al-Shobaki, Palestinian ambassador to Saudi Arabia, told Reuters that Abbas and Meshaal had agreed to discuss forming a national unity government “that can end the siege”.

Abbas and Meshaal failed to settle their differences over a unity government in a meeting last month in Damascus.

Hours after Fatah gunmen set Islamic University, a Hamas stronghold, ablaze, explosions and fires ripped through the nearby campus of al-Quds University, a Fatah bastion. Hamas denied any involvement in the al-Quds attack.

Ashraf Reziq, who lives a block from the Islamic University, said nobody feels safe. “Gaza has turned into a city of ghosts,” the 22-year-old said. “No one is in the streets … If we have one thing at all, it is fear.”

After talks with Egyptian mediators, Hamas and Fatah leaders promised to revive their ceasefire deal. Abbas and Palestinian Prime Minister Ismail Haniyeh of Hamas both called for calm.

Hamas defeated the once-dominant Fatah in elections a year ago. Since taking over the government in March, Hamas has struggled to govern under the weight of U.S.-backed sanctions imposed over its refusal to recognise Israel, renounce violence and accept existing interim peace deals with the Jewish state.

During Friday’s fighting, Hamas attacked a base used by Abbas’s presidential guard, pounding it with mortar bombs before taking it over. Hamas said it seized weapons at the base before setting it on fire.

Hamas commandeered a police station dominated by Fatah in Gaza City. In northern Gaza, Hamas seized a headquarters of the Preventive Security Service, a police station, intelligence offices and a base used by an elite Abbas force.

Six members of Abbas’s presidential guard and two members of his intelligence service were among the dead, including the head of Abbas’s General Intelligence in northern Gaza. “I call upon everyone, regardless of their affiliation, to stop this bloodletting,” Abbas said in the West Bank city of Ramallah.

A fragile truce between the rival factions broke down on Thursday after Hamas gunmen ambushed what the Islamist group said was a convoy carrying military equipment to Abbas’s presidential guard in Gaza.

In the occupied West Bank, the Israeli army said it killed two Palestinian gunmen, identified by Palestinian officials as members of the Preventive Security force.