The US has denied a media report that chemical weapons have been used in the Syrian conflict.

It reiterated that if Syrian President Bashar al-Assad's government did resort to using such weapons, it would be held to account.

"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 programme," White House National Security Council spokesman Tommy Vietor said.

He was responding to a report in foreign policy magazine that a secret US State Department cable had concluded the Syrian military likely used poison gas against its own people in a deadly attack last month.

"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," Mr Vietor said.

The foreign policy report said US diplomats in Turkey had investigated claims that Mr Assad's forces used chemical weapons, and had made a compelling case that poison gas had indeed been used.

The results of that probe were outlined in a cable that was signed by the US consul general in Istanbul and sent to the State Department in Washington last week, the magazine said.

Russian Foreign Minister Sergei Lavrov said last month that Mr Assad's government 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.

According to the United Nations, more than 60,000 people have died in the 22-month long conflict between Mr Assad's  forces and the opposition battling to end decades of his family's rule.