Obama also said Russia's offer to work with Damascus to put its chemical weapons under international control is “potentially positive”.

"I think you have to take it with a grain of salt initially," he told "NBC Nightly News" in an interview. "This represents a potentially positive development," he said on Monday, adding that Secretary of State John Kerry would explore with Russia how serious the offer is.

Obama was speaking in one of six television interviews he gave on Monday to make his case that Congress should grant him authority to take action against Syria in response to an alleged August 21 chemical weapons attack that killed more than 1,400 people.

The White House is making an all-out effort to win congressional support for a limited military strike against Syria, holding numerous briefings for lawmakers. The president plans to address the nation on television Tuesday night and is due to speak to senators of both parties on Capitol Hill during the day.

Obama told CNN that any diplomatic effort must be serious.

"And we don't want just a stalling or delaying tactic to put off the pressure that we have ... right now," he said.

"We have to maintain this pressure, which is why I'll still be speaking to the nation tomorrow about why I think this is so important," he added.

The president said a breakthrough on control of Syrian chemical weapons would not solve "the underlying terrible conflict inside of Syria. But if we can accomplish this limited goal without taking military action, that would be my preference."

Obama said he has not made up his mind whether he will forge ahead with military action if Congress votes his proposal down.

"It's fair to say that I haven't decided," he told NBC.

Obama faces an uphill struggle to win approval from lawmakers, and he acknowledged that he has doubts about how the vote will turn out.

"You know, I wouldn't say I'm confident," he said in his NBC interview.

The call for military action against Syria intensified after foreign-backed opposition forces accused the Syrian government of launching a chemical attack on militant strongholds in the suburbs of Damascus on August 21.

Damascus has strongly denied the accusations, saying the chemical attack was carried out by the militants themselves as a false flag operation.

NTJ/BA