The UN secretary-general says the Yarmouk Palestinian refugee camp near Damascus that has been overrun by Islamic State (IS) militants is "the deepest circle of hell" in Syria's four-year horror.
Ban Ki-moon told reporters on April 9 that "a refugee camp is beginning to resemble a death camp" for its estimated 18,000 residents.
He warned 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 said.
IS fighters overran much of the Yarmouk camp last week.
The secretary-general is demanding an end to the fighting, access for humanitarian aid and safe passage for people who wish to flee.
The Yarmouk camp was home to some 160,000 Palestinians before the Syrian conflict began in 2011, refugees from the 1948 war of Israel's founding.
The International Committee of the Red Cross called on April 9 for immediate access to Palestinian civilians trapped in the Yarmouk refugee camp.
"Humanitarian needs are growing by the day in the camp, which has been hard hit by four years of conflict and cut off from outside help for long periods," the Red Cross stated.
Amnesty International said at least 18 civilians had been killed during the past week as IS militants began their violent takeover of the camp.
The human rights group warned that the Syrian government has intensified its counterattack and is using highly inaccurate barrel bombs, raising the chances of higher civilian casualties.
Palestinian armed groups are also fighting, trying to push back the IS extremists, who have reportedly carried out beheadings in the southern parts of the camp they have captured.
"For civilians still trapped in Yarmouk, life is an agonizing struggle for survival. After enduring a crippling two-year-long government-imposed siege, now they are pinned down by sniper fire, fearing for their lives as shelling and aerial attacks escalate," Amnesty's Hassiba Hadj Sahraoui said.
With reporting by AP, dpa and Reuters