Washington (AFP) - US President Barack Obama says he does not regret his speech drawing a "red line" over Syria's use of chemical weapons, a phrase critics say symbolizes the US failure to act over the country's conflict.

Obama made the comment in 2012 about possible US military action in Syria, saying "a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized."

In what was billed as his last network interview, on CBS News program "60 Minutes" broadcast on Sunday -- less than a week before his term ends Friday -- Obama confirmed that he had ad-libbed the phrase, which wasn't in the text of his speech.

"I don't regret at all saying that if I saw (Syrian President) Bashar al-Assad using chemical weapons on his people that that would change my assessments in terms of what we were or were not willing to do in Syria," he said.

Challenged by his interviewer Steve Kroft that "you didn't say that... you said -- you drew the red line," Obama declined to say whether he would take it back.

"I think made a bigger mistake if I had said, 'Eh, chemical weapons. That doesn't really change my calculus.'"

"I think it was important for me as president of the United States to send a message that in fact there is something different about chemical weapons," he added. "And regardless of how it ended up playing, I think in the Beltway, what is true is Assad got rid of his chemical weapons."

In 2013, the Syrian military used chemical weapons in an attack against rebel-controlled areas of Damascus, killing nearly 1,500 civilians, including more than 400 children.

Wrenching video showing people foaming at the mouth and suffering other effects of an apparent chemical attack caused outrage around the world.

But after it appeared the United States was preparing imminent air strikes against the Syrian government, Washington instead agreed to a last-minute, Moscow-brokered deal to send Syrian chemical weapons to Russia.

Critics blame the decision for humiliating the White House, encouraging Russia to launch its own air strikes in Syria -- which have shored up Assad and killed many civilians -- and emboldening Moscow to ratchet up its confrontation with Washington.