At least 115 children have been killed and 172 injured in Yemen since the conflict began last month, according to the UN's agency for the welfare of children.

A spokesman from UNICEF said on Friday at least 64 children who had died between March 26 and April 20 had been killed by the strikes.

"We believe that these are conservative figures," said UN official Christophe Boulierac.

Another 26 children had been killed by unexploded bombs and mines, 19 by gunshots, three by shelling and three by "unverified causes related to the conflict," the agency said.

Saudi Arabia and its Arab allies launched the air war at the end of March as Iranian-backed Houthi fighters swept across the country.

Yemen's children: 'When can I go out and play again?'

The World Health Organisation on Thursday said the overall death toll in Yemen had topped 1,000, and the UN's human rights agency said on Friday at least 551 of the people who died were civilians.

UNICEF, meanwhile, said that since March 26, at least 140 children had been recruited by armed groups.

The agency's representative in Yemen Julien Harneis said earlier this month that up to a third of fighters in the country were children.

"Hundreds of thousands of children in Yemen... continue to live in the most dangerous circumstances, many waking up scared in the middle of the night to the sounds of bombing and gunfire," Harneis said in a statement on Friday.

Humanitarian crisis

The spiralling conflict has fuelled a humanitarian disaster in a country that was already suffering from shortages before the latest fighting erupted.

The UN's World Food Programme warned on Friday that a full 12 million people in the country did not know where their next meal was coming from, a 13 percent increase since the conflict escalated in late March.

The agency said it was delivering food to more than 100,000 people sheltered around the southern port city of Aden.

"But we are struggling to reach people because of deteriorating security," a WFP statement said, adding that dire fuel shortages were also hampering the response.

The food agency said it hoped to provide emergency food aid to 2.5 million people from May to July.