DAMASCUS — United Nations officials warned on Wednesday of a potential “slaughter of innocents” unless aid and assistance reached a Palestinian refugee camp in Syria where thousands of civilians are trapped up in a vicious battle.

Some 18,000 civilians, including 3,500 children, are caught in the camp outside Damascus just a few miles from President Bashar al-Assad’s palace. The camp largely fell under “Islamic State” control last week and is surrounded by the Syrian army.

The Yarmouk camp, which was home to half a million Palestinians before the conflict began in 2011, has been held by anti-Assad insurgents and besieged by government troops since the early days of the war and many have already fled.

But as fighting intensified in and around the camp, the remaining refugees have been left without food, water and medical supplies prompting aid agencies to call for the warring parties to allow access for aid and evacuations.

“The level of inhumanity that Yarmouk has descended to is frankly unimaginable,” Chris Gunness, a spokesman for the U.N. Relief and Works Agency (UNRWA), said in a Skype interview from Jerusalem.

“The situation is absolutely desperate. We need urgently to have humanitarian access, which is why UNRWA is calling for all parties to exercise influence with their clients on the ground so that we can get into the camp.”

Gunness said since the fighting escalated a week ago, UNRWA aid convoys have not been able to enter the camp and the already dire situation has become more desperate with people left without water, food or medicine and the growing risk of disease.

Gunness said a few Palestinian charities had managed to get some supplies to the camp but otherwise people were cut off with the risk of disease rising daily.

“When you have public health system completely shot to pieces, when you have such terrible levels of food insecurity … disease is going to soar,” said Gunness.

“We have tragic pictures of children and others scooping up water out of holes in the streets.”

Gunness said UNRWA had not been a regular presence in the camp for at least two years so it was impossible to estimate the number of people now ill or the death toll.

He added that an estimated 94 civilians, including 43 women and 20 children, managed to leave the camp on Sunday and were provided with humanitarian support so there was no reason why more could not be helped to safety.

“For me it is unconceivable that the so-called civilized world can stand and watch what is going on in Yarmouk,” he said. “We are facing a potential slaughter of the innocents and the world cannot stand by and watch that happen.”