ADEN — The UN expressed concern on Wednesday for more than 40,000 displaced Yemenis who had sought refuge in second city Aden, only to find themselves caught in deadly fighting between troops and separatist militia.

The UN refugee agency (UNHCR) said it had been unable to distribute aid since the southern separatists overran most of the city at the weekend opening up a new front in Yemen's devastating three-year civil war.

"UNHCR emergency aid distributions and humanitarian assessments planned this week for vulnerable, displaced Yemenis have now been postponed and UNHCR humanitarian cargo remains at Aden port unable to be released," the agency said on Twitter.

"We are also particularly concerned for those newly displaced in Aden who have fled other areas in Yemen. More than 40,000 people fled to Aden and nearby governorates since December and we anticipate more displacement as people continue to flee from hostilities in the west coast."

The separatists, who had been in an uneasy alliance with the beleaguered government of President Abedrabbo Mansour Hadi, launched their assault in Aden on Sunday and swiftly overran his troops, laying siege to the presidential palace.

Aden has been the headquarters of Hadi's ministers since 2015, when Shiite rebels overran the capital Sanaa and much of the north.

Hadi himself is based in exile in Saudi Arabia, which has led a military intervention in support of his forces since 2015.

International charity Save the Children said on Tuesday that it too had been forced to suspend its work in Aden out of fear for the safety of its staff.

Even before the latest fighting, Yemen already faced the world's most serious humanitarian crisis, with some 8.4 million of its 22.2 million population at risk of famine, according to the UN. — AFP