Washington has directly accused either Syria or Russia of being behind airstrikes that destroyed a school complex in Idlib Province but Moscow has denied that the attacks even took place.
Dozens of people, many of them children, are thought to have died in the bombing on Wednesday 26 October, which Washington blamed on Russia or its allies in the Syrian regime. The death toll, first put at between 22 and 26, is now thought to be almost 40, with up to 120 wounded.
According to reports, the attack took place just as children were being sent home because of air raids in the area. At least one bomb is reported to have detonated at the school entrance, killing and maiming many children.
Graphic images on social media showed dead children and a child's severed hand holding a school bag. A local doctor who asked not to be identified said many of those killed were aged between six and 15.
"Out of 120 wounded, there were about 40 children and 30 to 40 women, and the rest were various ages, mostly parents, who went to find out what happened to their children and ended up getting hurt in follow-up airstrikes," he said, as quoted by The Guardian.
"They were all civilians. There were no terrorists or armed men or fighters."
UNICEF's executive director, Anthony Lake, said the attack was one of the worst to date in the bloody conflict. "This latest atrocity may be the deadliest attack on a school since the war began more than five years ago," said Lake.
"Children lost for ever to their families...teachers lost for ever to their students...one more scar on Syria's future. When will the world's revulsion at such barbarity be matched by insistence that this must stop?"
Former UK Prime Minister Gordon Brown, now United Nations Envoy for Education, has called for an investigation into the incident, which he said may amount to a war crime. White House spokesman Josh Earnest laid the blame firmly on the Syrians and their allies the Russians.
However Russia's defence ministry denied the incident took place, claiming to have analysed images from the scene to show no explosion had occurred.
"On Thursday, a Russian UAV was directed to the area, to conduct detailed digital photography," said a statement from Defence Ministry spokesman Major General Igor Konashenkov, as published by RT.
"As can be seen from the photos taken by the drone, there are no signs of damage to the roof of the school, or craters from airstrikes around it."
Konashenkov went on to blame local civil defence unit, the Syrian White Helmets, for intentionally misleading UNICEF.
"Our evidence points to UNICEF falling victim to another scam from the White Helmet fraudsters," said Konashenkov. "Before making resonant statements, UNICEF should check its sources more carefully, to protect the reputation of the respected organisation."