A top US official on Wednesday categorically denied a report that according to a leaked State Department cable Syria had used chemical weapons in December.

"At the time we looked into the allegations that were made and the information that we had received, and we found no credible evidence to corroborate or to confirm that chemical weapons were used," State Department spokeswoman Victoria Nuland said.

She was responding to a report in online magazine Foreign Policy, which said a leaked State Department cable by US diplomats in Turkey made a "compelling case" that President Bashar al-Assad's forces had used poison gas.

"That report from Foreign Policy did not accurately convey the anecdotal information that we had received from a third party regarding an alleged incident in Syria in December," Nuland told journalists.

But she added that Washington, which has already warned Assad not to unleash chemical arms as it battles opposition rebels, would continue to keep a sharp eye on "Syria's proliferation-sensitive materials and facilities."

"We have been absolutely consistent and clear, from the president on down, there are red lines; we have a red line with regard to (the) use of chemical weapons or their proliferation," Nuland said.

"If the Assad regime makes the tragic mistake of using chemical weapons or fails to meet its obligations to secure them, there will be consequences, and the regime will be held accountable."

The United States was continuing to compare information and monitor Syria's chemical weapons stockpiles with other countries around the world, she added.