The heads of three U.N. agencies urged the Saudi-led military coalition to lift its blockade of Yemen, warning that "untold thousands" would die if it stayed in place.

The coalition closed all air, land and sea access to Yemen on Nov. 6 after the interception of a missile fired toward the Saudi capital, saying it had to stem the flow of arms from Iran to its Houthi opponents in the war in Yemen.

Yemen has 7 million people on the brink of famine, but without the reopening of all ports, that number could grow by 3.2 million, the heads of the World Food Program, UNICEF and the World Health Organization said in a joint statement.

"The cost of this blockade is being measured in the number of lives that are lost," the statement from David Beasley, Anthony Lake and Tedros Adhanom ­Ghebreyesus said. "Together, we issue another urgent appeal for the coalition to permit entry of lifesaving supplies to Yemen in response to what is now the worst humanitarian crisis in the world."

U.N. Secretary General António Guterres wrote to Saudi U.N. Ambassador Abdallah al-Mouallimi to warn him that the blockade was "already reversing the impact of humanitarian efforts," U.N. spokesman Stéphane Dujarric said Thursday.

"The secretary general is very much disappointed we've not seen a lifting of the blockade. The secretary general and his humanitarian team are heartbroken at the scenes we're seeing from Yemen," Dujarric told reporters.

Saudi Arabia has since said that aid can go through "liberated ports" but not Houthi- ­controlled Hodeidah, the conduit for the vast bulk of imports into Yemen.

For months, the United Nations has warned that the closure of Hodeidah would dramatically escalate the crisis.

As of Wednesday, 29 vessels, with 300,000 tons of food and 192,000 tons of fuel, had been blocked, while U.N. ships carrying $10 million of health and nutrition supplies and 25,000 tons of wheat were waiting to berth at Hodeidah, according to another U.N. statement.

"Without fuel, the vaccine cold chain, water supply systems and waste water treatment plants will stop functioning. And without food and safe water, the threat of famine grows by the day," the U.N. agency heads said.

At least 1 million children are at risk if a fast-spreading diphtheria outbreak is not stopped in its tracks, and the lives of 400,000 pregnant women and their babies are threatened because of the lack of medicine.

There is also the risk of a renewed flare-up of cholera, which was on the wane after the most explosive outbreak ever recorded — with more than 900,000 cases and 2,200 deaths in the past six months.

The number of new cholera cases has declined for the past eight weeks, Sherin Varkey, UNICEF deputy representative in Yemen, said in a telephone interview from Sanaa on Thursday.

The government-controlled port of Aden has partially reopened for limited shipments of aid, while Sanaa airport remains closed for commercial and humanitarian flights.Humanitarian access is inadequate as "needs are so huge," he said.

"Our worry is with the fuel shortage this could affect and even we assume reverse the declining trend (in cholera)," he said.