The plight of civilians in the Yarmouk refugee camp in the Syrian capital is an "epic test" for the international community's integrity, UN Secretary-General Ban Ki-moon said Thursday.

"In the horror that is Syria, the Yarmouk refugee camp is the deepest circle of hell," Ban told reporters at UN headquarters in New York. "A refugee camp is beginning to resemble a death camp."

He urged the global community to take concerted action to prevent a massacre of refugees and to "refuse to tolerate the intolerable" as refugees now face "a double-edged sword," with armed extremist groups inside and government forces outside.

The camp, which had been besieged by government troops since 2012, has been engulfed by intense fighting since the beginning of April when Daesh militants reportedly entered the camp.

The camp houses about 18,000 Palestinian refugees, including nearly 3,500 children.

Syria has been gripped by constant fighting since the regime of Syrian President Bashar al-Assad launched a violent crackdown in response to anti-government protests in March 2011, triggering a conflict that has killed more than 210,000, according to the UN.

The unrest created by the civil war paved the way for extremist movements, such as Daesh, to gain a foothold in the region.

The UN Security Council on Monday demanded humanitarian access to the camp to provide life-saving assistance and ensure the safe passage and evacuation of civilians.

Regarding the situation in Yemen, Ban said Saudi Arabian-led air strikes "have turned an internal political crisis into a violent conflict that risks deep and long-lasting regional repercussions."

He said the humanitarian crisis in the impoverished country had been one of the worst in the world even before the latest conflict, which started last September when pro-Houthi forces overran the capital of Sanaa, from where they have since sought to extend their influence to other parts of the country.

"The crisis has only multiplied in recent days," he said, with ordinary Yemeni families now struggling for the most basic needs such as water, food, fuel and medicines.

Since March 26, Saudi Arabia and several Arab allies have pounded positions held by Houthi rebels.

Riyadh says the campaign is in response to appeals by Yemen's UN-backed President Abd Rabbuh Mansour Hadi-who is currently in Saudi Arabia-to "save the [Yemeni] people from the Houthi militias."

Some Gulf States accuse Shiite Iran of supporting Yemen's Houthi insurgency.

Ban said a UN-brokered political dialogue remain the best chance to preserve the country's unity and territorial integrity.