Delivery of humanitarian aid to besieged areas of Syria continues to be blocked, and severely malnourished children will die if they are not reached soon, United Nations officials said Thursday.

“May was supposed to be a good month,” U.N. coordinator Jan Egeland told reporters in Syria. But of a million people in critical need of assistance, he said, aid organizations have reached only 160,000 of them as fighting has continued and the Syrian government, in some places, has continued to refuse access as a three-month-old cease-fire has fallen apart.

“The situation is horrendously critical,” said Egeland, who cited in particular the southern Damascus suburbs of Darayya and Moadamiya, as well as the al-Waer district of Homs.

The United States, Russia and other international stakeholders in the Syrian conflict agreed last week that if access could not be provided for road convoys by June 1, they would begin airdrops of aid.

Organized by the World Food Program, air operations require permission from the Syrian government, whose forces are responsible for most of the sieges, said Staffan de Mistura, the U.N. special envoy for Syria. If the government refuses, he said, “I will personally raise it” with the United States and Russia “to actually ensure that they find a way” to comply with their own decision to guarantee the deliveries.

Airdrops of food and other assistance started last month in Deir al-Zour, where 700 tons of aid — enough for 110,000 people for one month — has been delivered to the town occupied by Syrian government forces but surrounded by the Islamic State. De Mistura described it as “a joint operation with Russian pilots, WFP planes, U.S. funding and cooperation, U.S. parachutes and the contributions from countries like Netherlands, like Germany, like many others.”

The Syrian government, he said, has also “authorized and cooperated with the Iranian military authorities for airdrops or airlifts by helicopters of Iranian military to the two locations of Kefraya and Foua,” in Idlib province in the northwest.

“So the logic would be, since they are all Syrian civilians, Syrian people, the same type of authorization would be expected to be delivered for other places where Syrian civilians are present,” de Mistura said.

But Deir al-Zour is isolated in eastern Syria, far from the populated western cities where the Russian- and Iranian-backed government and U.S.-backed opposition rebels are locked in a civil war. It is unclear how the outside powers would cooperate in overflying and dropping aid in the contested areas.

Fighting has continued in those areas and beyond, despite a partial cease-fire initially declared under a February agreement between the United States and Russia. In and around Aleppo — where combat between the government and rebels has been particularly fierce, and government aircraft have struck repeatedly — U.N. aid officials “have to move from place to place to be able to survive,” Egeland said. “The U.N. hub has been repeatedly hit. And we have, in some places, been denied going there because the access route was not cleared, or was not safe and there was fighting in these places.”

Until the cease-fire began, Russia was regularly conducting air attacks in Syria, claiming that it was targeting forces of Jabhat al-Nusra, al-Qaeda’s affiliate there, and denying charges by the Obama administration and its allies that it was targeting opposition groups for the Syrian government. Last week, Russia announced that it would resume full-scale strikes May 25. On that day, however, it said it would delay those plans at the request of the United States.

The fighting has undercut U.N.-led political talks by the Syrian government and the opposition, which last month suspended its participation until there was improvement in the situation on the ground. De Mistura, who originally said he hoped to resume the talks this month, told the U.N. Security Council on Thursday that they would begin “as soon as feasible but certainly not within the next two [or] three weeks.”

Today's coverage from Post correspondents around the world