The United States accused the Syrian government on Tuesday of preventing humanitarian aid deliveries to all besieged and hard-to-reach areas of the country as required in U.N. resolutions, even as a cease-fire that was meant to allow such deliveries continues.

U.S. Ambassador Samantha Power told reporters after a closed-door emergency briefing to the Security Council by the U.N. humanitarian chief that less food aid reached millions of people in besieged and hard-to-reach areas in March than in February, when the cease-fire began. She gave no figures.

Power said the situation in Daraya, which is close to Syria's capital, Damascus, and hasn't received "one crumb of U.N. food" since 2012, has generated "a lot of outrage and heartbreak."

Humanitarian chief Stephen O'Brien told the council last week that Syria's government hasn't approved aid for three besieged areas "mere minutes' drive away from U.N. warehouses in Damascus" — Duma, East Harasta and Daraya.

He said the U.N. has received reports that severe food shortages are forcing some people in Daraya to eat grass.

Power also held up a photo of a skeletal boy in Madaya, which recently received several aid deliveries. She said O'Brien described Tuesday how the U.N. tried to evacuate him for medical treatment but the Syrian government refused and the boy, who was around 14 years old, died on Monday.

O'Brien said on March 30 that desperately needed aid has reached only 30 percent of Syrians living in besieged areas and less than 10 percent in hard-to-reach areas this year, even with the cease-fire. He said many of the 4.6 million Syrians in need in these areas can't be reached because of insecurity and obstruction by combatants.

Power said the United States and other Security Council members are appealing to those with influence on the Syrian government to ensure that every area is reached.

She said the government has proven "susceptible to influence when the pressure is intense enough ... and it's going to take a very, very, very large push to change the trajectory for people who are this malnourished."

Her appeal was clearly aimed at Russia, Syria's closest ally.

Russia's deputy U.N. ambassador Vladimir Safronkov told reporters that under the cessation of hostilities agreement a humanitarian working group was created in Geneva, "and this is where we are working" on implementation of U.N. resolutions adopted in December and February calling for humanitarian access to all besieged and hard-to-reach areas.

"It's a better way to work together," Safronkov said.