The United Nations has launched an appeal for almost $275m (€254m) to provide humanitarian aid to 7.5 million people in Yemen over the next three months.
Fighting in the south of the country is intensifying, with air strikes targeting 18 of Yemen's 22 provinces.
About 150,000 people have been displaced by the fighting.
The number is 50% more than the previous UN estimate, the UN humanitarian agency OCHA said, citing local sources.
The agency said health facilities had reported 767 deaths from 19 March to 13 April.
UN humanitarian coordinator for Yemen, Johannes Van Der Klaauw said: "Thousands of families have now fled their homes as a result of the fighting and air strikes,
Ordinary families are struggling to access healthcare, 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 1 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."
Even before the current conflict, Yemen was in a large-scale humanitarian crisis, with 15.9 million people - 61% of the population - estimated to require some kind of humanitarian aid.
The UN calculates it needs $273.7m (€254.2m) to provide what Yemen needs.
The largest part, $144.5m (€134.2m), aims to ensure food security for 2.6m people.
Yemen already had 10.5 million people classed as "food insecure" in December 2014.
That number has now risen to 12m and is expected to rise further as the fighting continues.
An estimated 100,000 tonnes of food are needed each month, but current World Food Programme stocks are limited to 37,000 tonnes, the appeal document said.