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 a 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-years civil war.

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

UN said it is also particularly concerned for those people who are newly displaced in Aden who have fled to other areas in Yemen.