A U.N. Security Council resolution that passed with great fanfare nearly two months ago has done little to facilitate the shipment of humanitarian aid to a quarter of a million Syrians under siege in their country’s civil war, the top U.N. aid official said Wednesday.

“Far from getting better, the situation is getting worse,” Valerie Amos, the undersecretary for humanitarian affairs, said after briefing the council. “All parties” in Syria are guilty of violating “the most fundamental human rights” of civilians and disregarding “the basic tenets of international law,” she said.

The situation is particularly severe in Aleppo, where a U.N. team was unable over the past week to get aid to civilians cut off from assistance since the government began a military campaign in the fall to retake rebel-held areas.

Hundreds and perhaps thousands of civilians have been killed during a sustained government campaign of airstrikes, including so-called barrel bombs, which have leveled much of the city.

On Wednesday, Syrian warplanes bombed a school in Aleppo just as students were staging an art show, killing at least 19 people and injuring many more. At least 10 of the dead were children, according to the Britain-based Syrian Observatory for Human Rights.

The dire conditions in Aleppo and other cities in western Syria helped propel the approval in February of a U.N. resolution ordering all parties to allow the passage of humanitarian aid and pledging unspecified “further steps” if its demands were not met. It marked the first time Russia, the primary arms supplier of Syrian President Bashar al-Assad, had voted in favor of a U.N. measure on Syria since the conflict began more than three years ago.

But aid continues to be blocked by government and rebel forces in many areas, Amos said. An estimated 9 million of Syria’s 25 million people have fled their homes since the fighting began, according to U.N. figures; two-thirds of them have sought refuge in neighboring countries that have been overwhelmed by refugees. Of the 2.5 million displaced people in Syria, the United Nations estimates that 242,000 live in “besieged areas,” where less than 10 percent have received any kind of outside assistance.

“In the absence of progress on the political front,” Amos said, “we need to look at a range of ways of really getting a step change of humanitarian assistance on the ground.” Russia is expected to veto any measure calling for action against the Syrian government.

The missile that struck the Ain Jalout elementary school in the rebel-held east of Aleppo was among at least half a dozen bombs dropped as part of the latest battle between government and opposition forces for control of the city, making it unclear whether the school was targeted.

Activists’ videos posted on YouTube showed extensive damage to the school, part of which was demolished, including collapsed classrooms and rubble strewn through corridors with Disney characters painted on the walls.

Another activist video on YouTube showed more than a dozen bodies laid out in zipped bags. There was also a video showing remnants of the students’ artwork. One painting featured a skeleton hanging over piles of skulls, while others depicted birds as planes dropping bombs.

A recent rebel counteroffensive has intensified the fighting but not produced any clear direction to the nearly two-year-old battle for control of Syria’s biggest city and its commercial capital.

A day earlier, at least 14 students were killed in government-controlled Damascus when mortars struck a renowned Islamic school in the heart of the capital’s Old City.

Sly reported from Beirut.