The United Nations on Friday appealed for $1.6 billion to help the 21 million people in need of aid in war-torn Yemen, warning of a "looming catastrophe".

The United Nations on Friday appealed for $1.6 billion to help the 21 million people in need of aid in war-torn Yemen, warning of a "looming catastrophe".

The money is needed to address the "constantly increasing humanitarian needs in Yemen" until the end of 2015, Jens Laerke, spokesman for the UN's humanitarian agency, told reporters in Geneva.

He added that more than "21 million people, or 80 percent of the population, is now estimated to be in need of some form of humanitarian aid and/or protection."

The appeal came as Yemen's warring sides entered a fifth and likely final day of what appeared to be deadlocked consultations with the UN's special envoy for Yemen, Mauritanian diplomat Ismail Ould Cheikh Ahmed, in Geneva.

UN Under Secretary for Humanitarian Affairs Stephen O'Brian launched the aid appeal in Geneva on Friday, revising the figure up from a combined $1.02 billion requested in December and April.

At the launch, O'Brian warned that "a looming humanitarian catastrophe is facing Yemen," Laerke said.

"People across the country are struggling to feed themselves and their families and basic services are collapsing in all regions of the country."

Only $200 million of the needed cash has so far been received, the UN said, adding that Saudi Arabia's pledge last month of $540 million had yet to materialize.

Yemen has been since March 26 under brutal aggression by Saudi-led coalition. Thousands have been martyred and injured in the attack, with the vast majority of them are civilians.

Riyadh launched the attack on Yemen in a bid to restore power to Yemen’s fugitive president Abd Rabbuh Mansour Hadi who is a close ally to Saudi Arabia.