The UNations has launched an appeal for £180 million to aid 7.5 million people in Yemen over the next three months, as fighting intensifies in the south and air strikes pound 18 of Yemen's 22 provinces.

About 150,000 people have been displaced, 50 per cent more than the previous U.N. estimate, the UN humanitarian agency OCHA said.

The agency said health facilities had reported 767 deaths from March 19 to April 13, almost certainly an underestimate.

"Thousands of families have now fled their homes as a result of the fighting and air strikes," the UN humanitarian coordinator for Yemen, Johannes Van Der Klaauw, said.

"Ordinary families are struggling to access health care, water, food and fuel - basic requirements for their survival."

The fighting had destroyed, damaged or disrupted at least five hospitals, 15 schools, Yemen's three main airports, two bridges, two factories and four mosques, as well as markets, power stations and water and sanitation facilities, OCHA said.

"Public water services covering one million people are at serious risk of collapse," the UN appeal document said. "Hospitals are overwhelmed with casualties, including people who have been direct victims of violence and those suffering severe burns from explosions."