Yemen, long one of the world's poorest nations, has seen its food security deteriorate dramatically since its civil war escalated two years ago after the intervention of a Saudi-led coalition.
"We are deeply concerned that Yemen is on the brink of famine," Bettina Luescher, a spokeswoman for the UN's World Food Programme, told reporters in Geneva.
"Out of the 22 (provinces), seven are in emergency phase four, and that is one level before declaring a famine," she said.
And even those lucky enough to get aid are not receiving all the nutrients they need, Luescher said, because full rations cannot be afforded.
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"Lack of funding, the ongoing conflict, restricted movement of humanitarian aid workers are the major obstacles to get food and other assistance to the people," she said.
The Yemen conflict has left more than 7,400 dead and 40,000 wounded since the Saudi-led coalition joined the government side against Iran-backed rebels in March 2015, according to UN figures.
The conflict has dramatically affected food supply, with around 60 percent of Yemen's population now considered to be struggling to find enough, up 20 percent from a year ago, Luescher said.
"That is why we are so concerned about the fact that the fighting is going on, that the ports often can't operate, that bridges are being blown up, that trucks cannot go to areas," she said.
A famine is declared when 20 percent of the population faces "extreme food shortages with limited ability to cope", World Health Organization spokesman Christian Lindmeier explained.
Yemen is one of four countries currently facing the risk of famine, alongside Somalia, South Sudan and Nigeria, with more than 20 million facing starvation, according to the UN.