The UN secretary-general says the Yarmouk Palestinian refugee camp near Damascus that has been overrun by Islamic State group fighters is "the deepest circle of hell" in Syria's four-year horror.

Ban Ki-moon told reporters Thursday that "a refugee camp is beginning to resemble a death camp" for its estimated 18,000 residents.

He warns that the armed fighters inside the camp and Syrian government forces outside it are a double-edged sword for the thousands trapped.

"We simply cannot stand by and watch a massacre unfold," Ban says.