The UN has launched an appeal for nearly $275m (£183m;€255m) of aid to help civilians caught up in Yemen’s worsening conflict.
About 150,000 people have been displaced by fighting, according to the UN, with 12 million short of food.
Meanwhile, al-Qaeda militants overran an arms depot in Yemen’s Hadramawt province, seizing heavy weapons.
A Saudi-led coalition has been bombing Yemen’s Shia Houthi rebels, who are advancing across the country.
The coalition is bombing 18 of Yemen’s 22 provinces, exacerbating an existing humanitarian crisis.
The UN said 731 people had been killed and 2,754 injured – many of them civilians – in three weeks between March and April. The numbers were likely to be an underestimate, it warned.
The UN Office for the Co-ordination of Humanitarian Affairs (Ocha) is now calling for $273.7 million in donations.
“Ordinary families are struggling to access healthcare, water, food and fuel – basic requirements for their survival,” said Johannes Van Der Klaauw, UN humanitarian coordinator for Yemen.
“To scale up assistance, we urgently need additional resources. I urge donors to act now to support the people of Yemen at this time of greatest need.”
Also on Friday, security officials said al-Qaeda militants overran Yemeni troops and took control of a massive weapons depot in the eastern Hadramawt province, consolidating their control of the region.
The militants reportedly seized dozens of tanks, rocket launchers, and small arms.
The attack came a day after the group captured a major airport in the region, as well as an oil terminal and the main military base.
The fighting in Yemen – between the Shia Houthi rebels, forces loyal to exiled President Abdrabbuh Mansour Hadi, and jihadists – has dramatically accelerated an existing humanitarian crisis.
Even before the current conflict, 15.9 million people – 61% of the population – were estimated to require some kind of humanitarian aid, according to the UN.
Air strikes have worsened the damage to the country’s infrastructure, with at least five of Yemen’s hospitals now destroyed or badly damaged.
The UN also said it had received reports of serious human rights violations.