In a worsening situation since early February, fighting on several fronts in Yemen's oil-rich Marib Governorate has forced up to 15,000 people to flee their homes, according to the UN.

Roughly 60 per cent of the displaced are reportedly camping in informal settlements and crowded sites with services overstretched, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement on Friday.

Humanitarian groups are supporting both newly displaced and existing vulnerable internally displaced people, Xinhua news agency reported citing the statement as saying.

"Hostilities have also escalated in other parts of the country, including in Taiz, resulting in multiple civilian casualties and damage to civilian infrastructure," OCHA said.

"Across Yemen, 20.7 million people need humanitarian assistance, many of them at the brink of starvation.

"They are the ones who are paying the highest price in this war. The UN continues to call for an immediate end to hostilities and a nationwide ceasefire," it added.

Some 25 humanitarian agencies are operating, they include seven UN agencies, six international NGOs and 12 Yemeni non-profits.

Humanitarian assistance has a price tag, and OCHA said that on March 1, donors pledged $1.7 billion, less than half of what aid agencies say they need for this year.

The UN humanitarians are especially concerned about support for the Yemen Humanitarian Fund.

It supports dozens of partners and is one of the largest sources of direct funding for national NGOs.

Pledges of only $40 million were made, compared with $170 million contributed to the fund two years ago.