The White House early Wednesday cast doubt on, but did not outright deny, that a secret State Department cable had confirmed accounts of a chemical weapons attack orchestrated by the Syrian regime in the city of Homs in December 2012.

“The reporting we have seen from media sources regarding alleged chemical weapons incidents in Syria has not been consistent with what we believe to be true about the Syrian chemical weapons program,” said Tommy Vietor, a National Security Council spokesman, in a statement released on Tuesday.

The comment came close on the heels of a report in Foreign Policy Magazine to the effect that investigations by US diplomats into a Syrian army attack in Homs on December 23, 2012, found that President Bashar Assad’s forces had used a chemical known as “Agent 15″ or “BZ.”

The conclusions were sent to Washington in a diplomatic cable signed last week by the US consul-general in Istanbul, Scott Frederic Kilner, the report said.

After the attack, rebels released video showing alleged victims gasping for breath and vomiting, raising speculation that Assad had ordered that chemical agents be used in the bitter civil war which has claimed some 60,000 lives in under two years.

The use of unconventional weapons by regime forces in Syria has the potential to broaden the conflict in the war-torn country. US President Barak Obama in August said that the deployment of chemical weapons would be a game-changer.

“We have communicated in no uncertain terms with every player in the region that that’s a red line for us, and that there would be enormous consequences if we start seeing movement on the chemical weapons front, or the use of chemical weapons,” he said at the time. “That would change my calculations significantly. That would change my calculus; that would change my equation.”

According to the Foreign Policy report, investigators spoke to doctors and activists before sending the cable with the damning conclusions. The report cited a US government official who had apparently seen the communication.

“We can’t definitely say 100 percent, but Syrian contacts made a compelling case that Agent 15 [a chemical weapon] was used in Homs on December 23,” the official was quoted as saying.

Wired Magazine’s national security blog, The Danger Room, explained that Agent 15 is a hallucinogen rather than a lethal compound. During the Cold War, the US military tested the agent on its own soldiers, but there are no previous reports to date of it being weaponized, the blog said. The Danger Room said Assad was not known to be in possession of the agent.

Vietor, the White House spokesman, reiterated Obama’s warning that the use of chemical weapons would provoke a response from the US.

“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.

However, last week Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, acknowledged that it would be nearly impossible to prevent the Syrian government from using its chemical weapons. He claimed that the US must instead rely on deterrence and continue warning Syria that using such weapons would be unacceptable.