Story highlights ISIS has seized control of large parts of the Yarmouk Palestinian refugee camp in Syria

An estimated 18,000 refugees are trapped between militant groups and regime forces

U.N.: "In the horror that is Syria, the Yarmouk refugee camp is the deepest circle of hell"

(CNN) They took Yarmouk by storm, a sea of masked men flooding into the streets of one the world's most beleaguered places.

Besieged and bombed by Syrian forces for more than two years, the desperate residents of this Palestinian refugee camp near Damascus awoke in early April to a new, even more terrifying reality -- ISIS militants seizing Yarmouk after defeating several militia groups operating in the area.

"They slaughtered them in the streets," one Yarmouk resident, who asked not to be named, told CNN. "They (caught) three people and killed them in the street, in front of people. The Islamic State is now in control of almost all the camp."

An estimated 18,000 refugees are now trapped inside Yarmouk, stuck between ISIS and Syrian regime forces in "the deepest circle of hell," in the words of U.N. Secretary-General Ban Ki-moon.

Yarmouk, the largest Palestinian refugee camp in Syria, was formed in 1957 to accommodate people fleeing the Arab-Israeli conflict.

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