BEIRUT (AP) — The leader of a powerful al-Qaida-linked group in Syria gave a rival breakaway group a five-day ultimatum to accept mediation by leading clerics to end infighting or be "expelled" from the region.

The ultimatum announced in an audio recording by the leader of the Nusra Front aims to end months of deadly violence between the Islamic State of Iraq and the Levant and other Islamic factions. The fighting has killed hundreds of people since the beginning of the year and is undermining their wider struggle against President Bashar Assad.

It comes two days after the killing of Abu Khaled al-Suri, who had acted as al-Qaida chief Ayman al-Zawahri's representative in Syria. Rebels and activists believe he was assassinated by two suicide attackers from the Islamic State of Iraq and the Levant.

Both the Nusra Front and the Islamic State of Iraq and the Levant are considered terrorist organizations by the United States.

Al-Zawahri has named the Nusra Front al-Qaida's branch in Syria and broken ties with the Islamic State, which has increasingly clashed with rebel brigades in opposition-held areas of Syria. The Islamic State has angered other factions with its brutal tactics and campaign to Islamize areas under its control in the northeast.

More than 2,000 people have been killed in the fighting between the Islamic State and rebel groups, including the Nusra Front.

Abu Mohammed al-Golani, the Nusra Front leader, suggested in the audio recording arbitration by clerics to stop the infighting. He warned the Islamic State that it would be driven from Syria and "even from Iraq" if it rejected the results of arbitration. He did not elaborate on how his group might do that.

"We are waiting for your official answer within five days of issuing this statement," al-Golani said in the audio message posted on militant websites. "By God, if you reject God's judgment again, and do not stop your arrogant overlording over the Muslim nation, then (we) will be forced to launch an assault against this aggressive, ignorant ideology and will expel it, even from Iraq."

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Al-Golani suggested the arbitration be conducted by three senior al-Qaida ideologists, including two imprisoned in Jordan and one imprisoned in Saudi Arabia. He did not say how they will handle the arbitration while they are in detention.

Syria's conflict began with largely peaceful protests in March 2011 and gradually descended into civil war. Islamic extremists including foreign fighters have joined the war against Assad, playing an increasingly powerful role in the effort to topple him.

More than 140,000 people have died in the past three years, according to opposition activists.

On Tuesday, the chief of the United Nations relief agency supporting Palestinian refugees spoke of a rare visit he paid a day earlier to the besieged Palestinian camp of Yarmouk in Damascus.

Filippo Grandi, the Commissioner General of UNRWA, said the extent of damage to the refugees' homes in Yarmouk was shocking.

"The devastation is unbelievable. There is not one single building that I have seen that is not an empty shell by now," he said in neighboring Beirut.

The state of those still in the camp was even more shocking.

"It's like the appearance of ghosts," he said of the people coming from within Yarmouk near a distribution point he was allowed to reach.

"These are people that have not been out of there, that have been trapped in there not only without food, medicines, clean water — all the basics — but also probably completely subjected to fear because there was fierce fighting

Grandi welcomed last week's U.N. Security Council resolution calling for immediate access for humanitarian aid to all areas of Syria. He said that the resolution, unanimously adopted by the Security Council, "gives us a tool to argue in favor of access that is stronger than any other tool we've ever had before in Syria."

He said it's too early to say what effect the resolution has had on the ground, but that "everybody has to comply." Both sides have hindered access in the past, he added.

UNRWA shipments to Yarmouk were cut for months, leaving residents to suffer from crippling shortages of food and medicine. Since last month, small shipments resumed, although they remain intermittent.

More than 100 people have died in in the area since mid-2013 as a result of starvation and illnesses exacerbated by hunger or lack of medical aid, according to U.N. figures.

Yarmouk, located in southern Damascus, is the largest of nine Palestinian camps in Syria. Since the camp's creation in 1957, it has evolved into a densely populated residential district just five miles (eight kilometers) from the center of Damascus. Several generations of Palestinian refugees have lived there.

Grandi said around 18,000 of the camp's original 160,000 Palestinian refugees are still inside Yarmouk.

The most vulnerable Syrian refugees in neighboring countries are also at risk of hunger and sickness, said the U.N.

In neighboring Lebanon, nearly 2,000 Syrian refugee children under five years old are at risk of dying from hunger and need immediate help to live, the U.N. children's agency said in a report issued Tuesday.

"Malnutrition is a new, silent threat among refugees in Lebanon, linked to poor hygiene, unsafe drinking water, diseases, lack of immunization, and improper feeding practices of young children," said UNICEF representative Annamaria Laurini.

Nearly half the at-risk children live in makeshift tents in the eastern Lebanese Bekaa Valley that barely offer shelter from the cold, have no power or running water.

There are nearly 1 million Syrian refugees in Lebanon.

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Youssef reported from Cairo. Associated Press writers Barbara Surk in Beirut and Albert Aji in Damascus contributed.