As many as 7 million people in Yemen are acutely malnourished or do not know how they will get their next meal, according to the UN's aid chief.

Stephen O'Brien, undersecretary general for humanitarian affairs and emergency relief, spoke Wednesday from Yemen in a conference call with journalists in New York. O'Brien, who has been in Yemen since Sunday, said about 80 per cent of the people affected are in the north of the country and conditions are worsening. Efforts to support the starving population with relief supplies need to be reinforced, O'Brien said, adding that during his trip he has been denied access to combat areas. So far it has only been possible to reach 3-4 million of the estimated 7 million affected with only the most basic supplies every second month, he said.

Yemen is a poor country on the Arabian peninsula and it suffered from food and water shortages even before civil war broke out nearly two years ago.

Last year, U.N. agencies including WHO, received less than 60 percent of their appeal for $182 million to support Yemen's health sector. So far this year's appeal for $322 million for healthcare has been less than 1 percent funded, according to the U.N. Financial Tracking Service. WHO said only 45 percent of Yemen's health facilities were fully functional and accessible.

Highly specialized medical staff, including intensive care doctors, psychiatrists and foreign nurses, have left the country, and the health workers that have remained have not received regular salaries since September, the agency said.

Lack of money has forced Al-Tharwa hospital in the Red Sea port of Hodeidah - the main functioning health facility in the region - to stop providing food to its patients.

"There are acute shortages of certain medicines and we need more fuel to ensure the hospital has electricity," hospital director, Khaled Suhail, said in a statement, adding that with no funds to cover operational costs, he did not know if the hospital would be open in a month's time.

WHO said nearly 4.5 million Yemenis, including 2 million children, need assistance in treating or preventing malnutrition - a 150 percent increase since late 2014. "We urgently need resources to help support the health system as a whole, and are calling on donors to scale-up their support before more innocent lives are lost unnecessarily," Zagaria said.

Yemen, on the southern edge of the Arabian Peninsula, has been in the midst of a civil war since September 2014 when Shiite Houthi rebels swept into the capital of Sanaa and overthrew President Abed-Rabbo Mansour Hadi's internationally recognized government. In March 2015, a Saudi-led coalition of Arab countries began a military campaign against Houthi forces, saying its mission served in part as a counterbalance to Iran's influence with the Houthis following its nuclear deal with world powers.