An international aid group says an estimated 130 children or more die every day in war-torn Yemen from extreme hunger and disease.
Save the Children said late on Wednesday a continuing blockade by the Saudi-led coalition fighting Yemen's Shiite rebels is likely to further increase the death rate. It says over 50,000 children are believed to have died in 2017.
Saudi Arabia blocked Yemen's ports after a rebel missile attack near Riyadh earlier in November. It said Monday the coalition would lift the blockade after widespread international criticism.
On Thursday, the leaders of the World Health Organization, the U.N. children's agency and the World Food Program issued a joint appeal for the easing of the blockade.
"While the Saudi-led military coalition has partially lifted the recent blockade of Yemen, closure of much of the country's air, sea and land ports is making an already catastrophic situation far worse," they said.
"The space and access we need to deliver humanitarian assistance is being choked off, threatening the lives of millions of vulnerable children and families."
The Saudi-led coalition went to war against the rebels, known as Houthis, in March 2015 on behalf of Yemen's internationally recognized government. But the coalition has made little progress, and the rebels still control much of northern Yemen, including the capital, Sanaa.
The war has killed more than 10,000 people and displaced 3 million. Yemen was the Arab world's poorest country even before the conflict began.
The U.N. officials said more than 20 million people, including 11 million children, are in need of urgent assistance, with 7 million totally dependent on food assistance. The U.N. has called it the "worst humanitarian crisis in the world."
"Even with a partial lifting of the blockade, the World Food Program estimates that an additional 3.2 million people will be pushed into hunger. If left untreated, 150,000 malnourished children could die within the coming months," the officials said.
Later Thursday, the U.N. spokesman said Secretary-General Antonio Guterres has written to Saudi Arabia's U.N. ambassador, saying the kingdom's failure to reopen key airports and sea ports in Yemen is already reversing humanitarian efforts to tackle the crisis in the impoverished country.
Stephane Dujarric said Guterres welcomes the reopening of the port in the city of Aden but that this "will not meet the needs of 28 million Yemenis."
Dujarric said the United Nations is urging the Saudi-led coalition to resume U.N. humanitarian flights to Aden and Sanaa, and to reopen the ports of Hodeida and Salif for food and medical deliveries.
Like the capital, Sanaa, Hodeida and Salif are in rebel-held territory.