Evidence was growing yesterday that Bashar Al Assad's regime may have used chemical weapons against the Syrian people.
US diplomats in Turkey made a "compelling case" that regime forces used the poison gas Agent 15 in the western city of Homs on December 23, according to a confidential US state department cable leaked to the magazine Foreign Policy.
In a carefully phrased response, the US National Security Council spokesman Tommy Vietor said reports of such an attack were "not consistent with what we believe to be true about the Syrian chemical weapons programme".
Josh Rogin, a reporter with Foreign Policy, described the comments by Mr Vietor as a "non-denial denial".
He said on Twitter that the White House response "doesn't say what they believe happened in Homs", and the US "may be trying to say they don't think Syria has Agent 15, but the point is whether any CW were used".
The Assad regime has given assurances that it will not use chemical weapons, but countries in the region have long feared that this might happen.
Jordan has taken precautions along its border against the possibility that Syria may use chemical weapons.
"Of course we have to be worried," said the prime minister, Abdullah Ensour. "We cannot take for granted the assertions that Syrian officials are making."
The state department cable was said to have been based on interviews with activists, doctors and defectors and to represent one of the most comprehensive US efforts to date to investigate claims by the Syrian opposition that the regime had used chemical weapons in Homs.
Referred to as "the capital of the revolution" by the opposition, the city has been repeatedly attacked by regime forces.
Foreign Policy spoke to doctors who treated victims in Homs on December 23 who said the poison gas, thought to be Agent 15, killed five people and affected about 100 others.
Doctors and activists have circulated videos of the victims on social media and YouTube to draw international attention to what they claim is the Assad regime's use of chemical weapons.
US officials said last month that Syria appeared to have taken steps towards mixing and deploying chemical weapons, but after a flurry of media reports and stern warnings from Washington and Russia the regime appeared to have backed off.
According to UN figures, more than 60,000 people have been killed in the violence in Syria since the outbreak of protests against Mr Al Assad's rule.
The US president would take a strong stance against Syria if chemical weapons are deployed, said Mr Vietor.
"The president was very clear when he said that if the Assad regime makes the tragic mistake of using chemical weapons, or fails to meet its obligation to secure them, the regime will be held accountable," he said.
Russia's foreign minister Sergei Lavrov said last month that the Assad regime had concentrated its chemical arsenal in two depots in an effort to make them more secure. Mr Lavrov also played down the chances that they would used against rebel fighters.
The violence showed no sign of abating yesterday as suicide bombers drove vehicles packed with explosives into security targets in Idlib.
There were conflicting reports about the number of explosions but state-run media and activists said at least 22 people had been killed.
Rebels control vast areas of the Idlib countryside, but the city itself is controlled by the regime.
The death toll from twin blasts that tore through the campus of Aleppo University on Tuesday has risen to 87, a watchdog said yesterday.
The Syrian Observatory for Human Rights said the death toll could rise even further because medics have collected unidentified body parts and some of the more than 150 wounded are in critical condition. Syria's ministry of higher education suspended classes and exams at all Syrian universities yesterday "in mourning for the souls of the heroic martyrs who were assassinated by the treacherous terrorist hand", the state news service reported.
* Reporting by Reuters, Agence France-Presse, the Associated Press and Bloomberg News