Without the assistance, however, "many people will predictably die from hunger, livelihoods will be lost, and political gains that have been hard-won over the last few years will be reversed".
Stephen O'Brien said this while addressing UN Security Council and asked to protect humanity from destruction; many people will face starvation and famine.
Mr O'Brien said that is now three million more chronically hungry people in Yemen than in January. But why are they in such dire need? In early March, about 110 people died in a two-day period.
The majority of those in need are in Yemen, said the body.
O'Brien said the warning call and appeal for action by U.N. Secretary-General António Guterres could not be understated.
"The starvation in South Sudan is man-made", he said.
He says the United Nations and its partners are ready to boost their response efforts but they need much more help with access and funding. Counted among them are 450,000 children who the agency says will face severe, acute malnutrition this year if they do not get help. "Meanwhile, the cholera outbreak that began in June 2016 has spread to more locations".
Turning to the countries he visited, the senior United Nations official said that, about two-thirds of the population (more than 18 million people) in Yemen needed assistance, including more than seven million severely food insecure, and the fighting continued to worsen the crisis.
A total of US$2.1 (A$2.8) billion are needed to reach 12 million people with life-saving assistance and protection in Yemen this year, according to O'Brien, who noted that just six percent of those funds have been received so far.
"What I saw and heard during my visit to Somalia was distressing - women and children walk for weeks in search of food and water", O'Brien said.
"They have lost their livestock, water sources have dried up and they have nothing left to survive on".
In the last starvation in Somalia, about six years ago, almost 260,000 people died.
But he said the UN's humanitarian partners have a larger footprint, better controls on resources, and a stronger partnership with the new government which recently declared the drought a national disaster. "Yemen, northern Nigeria and Somalia are also on the brink of famine", analysts said. We need access now.
In northeast Nigeria, more than 20,000 people have been killed during a seven-year uprising by the Boko Haram Islamic extremist group.
In February, a United Nations humanitarian coordinator said malnutrition in the northeast is so pronounced that some communities many young children have died.
World Bank Group President Jim Yong Kim today issued the following statement on the devastating levels of food insecurity in sub-Saharan Africa and Yemen: "Famine is a stain on our collective conscience".