Around 18,000 civilians remained trapped by Islamic State group militants in the Yarmouk refugee camp, a large settlement of Palestinian refugees in southern Damascus, Syria, the U.N. Relief and Works Agency reported. Food, water and medical supplies are low and there is no electricity in the camp, and those trapped there have been subjected to brutal acts at the hands of militants, activists said Sunday.

The Islamic State group -- also known as ISIS or ISIL -- has control of around 90 percent of the camp after launching offensives there Wednesday, Reuters reported. There are reportedly multiple factions and battalions of rebel groups fighting with each other and troops loyal to the Syrian government. The Syrian Observatory for Human Rights (SOHR), based in London, reported that helicopters dropped 13 barrel bombs “on areas in the Yarmouk camp which is witnessing clashes involving [the Islamic State group] against Aktaf Bait al-Maqdis and Islamic battalions.” Barrel bombs are crude improvised bombs typically made out of leftover oil drums filled with explosives. Because they cannot be aimed, they often kill civilians.

“Reports of kidnappings, beheadings and mass killings are coming out from Yarmouk, which is under a brutal campaign of murder and occupation,” said Dr. Saeb Erekat, a member of the executive committee of the Palestine Liberation Organization, CNN reported.

The SOHR also published a statement by Jabhat al-Nusra, an al-Qaeda affiliate in Syria, that said the group would refrain from fighting in Yarmouk “for the sake of the exhausted besieged area,” and would offer passage out of the camp for “all who do not want to fight.” Al-Nusra did, however, block a rival brigade from gaining access to Yarmouk to fight Islamic State group contingents on the basis of previous disputes, the statement said.

Yarmouk, which was established in 1957, is technically a camp, but has grown to become a fully established neighborhood of 140,000 people, complete with permanent concrete buildings and neighborhood amenities. The United Nations runs around two-dozen schools in Yarmouk.