Syria will "never, under any circumstances" use chemical weapons against its own people, a foreign ministry official insisted on Monday.

"In response to the statements of the US foreign minister, Syria confirms repeatedly it will never, under any circumstances, use chemical weapons against its own people, if such weapons exist," he said, quoted on state television.

After a New York Times report over detected movement of chemical weapons by the Syrian military, US Secretary of State Hillary Clinton issued a warning to Damascus on Monday.

"This is a red line for the United States," she said on the eve of a NATO meeting in Brussels. "Once again we issue a very strong warning to the (President Bashar al-) Assad regime."

A US official told the New York Times: "The activity we are seeing suggests some potential chemical weapon preparation."

The unnamed foreign ministry official in Damascus insisted his country was "defending its people by fighting terrorists linked to Al-Qaeda" and accused the United States and other "known countries" of backing the jihadists.

Syrian authorities and state media have labelled all armed opposition fighters as foreign-backed terrorists since the uprising against Assad's regime broke out in March 2011.

On October 1, Foreign Minister Walid Muallem accused Washington of raising fears over Syrian chemical weapons stockpiles as propaganda to back its case for the overthrow of the regime.

"These chemical weapons in Syria, if they exist -- and I emphasise if -- how is it possible that we would use them against our own people? It's a joke," he said in a television interview.

Damascus acknowledged for the first time in late July that it possessed chemical weapons. It threatened to use them if attacked by outside countries, but never against its own people.

According to experts, these stocks, which amount to hundreds of tonnes, date back to the 1970s and are the largest in the Middle East.