UN aid chief Stephen O'Brien met with heads of agencies to discuss the crisis in the impoverished Arabian Peninsula country.

"All agencies agreed to declare the level three for a period of six months," said UN spokesman Farhan Haq.

More than 21.1 million people - over 80 percent of Yemen's population - are in need of aid, with 13 million facing food shortages, AFP reports.

Under the emergency plan, the United Nations will try to reach 11.7 million people most in need.

"The health system is facing imminent collapse with the closure of at least 160 health facilities due to insecurity, lack of fuel and supplies," Haq said.

Saudi Arabia has been bombing Yemen in the last 99 days to bring its ally, fugitive president Mansour Hadi, back to power.

The airstrikes have so far claimed the lives of more than 4,781 civilians, mostly women and children.

According to a recent report by Freedom House Foundation, most of the victims of the deadly Al Saud campaign are civilians, including a large number of women and children.