President Barack Obama said Tuesday it was too early to say if a Russian plan to secure Syria's chemical weapons could forestall US air strikes, but vowed to give long-shot diplomacy a chance.

In a somber national address, Obama laid out his most precise, response yet to a chemical weapons attack on civilians in Syria last month, after days of confused diplomacy and contradictory messages from his administration.

He warned Americans tired of bloody engagements abroad, that for reasons of national security and moral decency, they could not simply look away after innocent children were gassed to death in an attack he blames on President Bashar al-Assad's regime.

"The images from this massacre are sickening: Men, women, children lying in rows, killed by poison gas," Obama said in the 16-minute speech.

"Others foaming at the mouth, gasping for breath. A father clutching his dead children, imploring them to get up and walk."

Obama vowed to keep the US Navy on station off the Syrian coast to keep up the pressure on Assad's regime while the diplomatic track evolves.

That warning, delivered in stern tones, may also have been meant as a sign to Russia that he would not stand for delaying tactics or endless diplomacy, which critics say is the inevitable result of Moscow's initiative.

Obama spoke on national television in the East Room of the White House after hurriedly reworking his address to respond to the suddenly emerging Russian plan to put Syria's chemical arms under international supervision.

"It's too early to tell whether this offer will succeed, and any agreement must verify that the Assad regime keeps its commitments," Obama said, from the same spot where he announced the death of Osama bin Laden in 2011.

"But this initiative has the potential to remove the threat of chemical weapons without the use of force, particularly because Russia is one of Assad's strongest allies," Obama said.

The president said he would dispatch Secretary of State John Kerry to Geneva to meet his Russian counterpart for talks on the crisis on Thursday.

And he pledged to also work personally with Russian President Vladimir Putin, with whom he has a frosty relationship as US-Russia ties plumb their worst depths since the Cold War.

The president, hammered for "weakness" by critics as he has tried to keep out of the Syrian civil war, said that it was only his threat of wielding force that had unleashed a sudden diplomatic opening.

The Russian plan, if it works, offers the president the chance to escape a perilous political spot, as he was at risk of ordering military action without support from Congress, the American public, or key allies like Britain or the United Nations.

He said that it was simply not an option for America not to respond to the August 21 chemical weapons attack, which Washington says killed 1,400 people.

"When dictators commit atrocities, they depend on the world to look the other way until those horrifying pictures fade from memory," Obama said.

"But these things happened.

"The facts cannot be denied. The question now is what the United States of America and the international community is prepared to do about it.

"Because what happened to those people, to those children, is not only a violation of international law, it's also a danger to our security."

Obama said he understood after the wars in Afghanistan and Iraq that Americans were weary of costly conflicts abroad -- and said he was more interested in ending wars than beginning new ones.

But he said that if America did not act, chemical weapons would be used again in flagrant violations of international law.

And he asked Americans to view the horrific videos of the attack outside Damascus.

Amid confusion about the extent of any US military strike, he warned that Assad would pay a heavy price if military action was used.

"The US military doesn't do pinpricks. Even a limited strike will send a message to Assad that no other nation can deliver," Obama said.

Obama also made clear he had also asked Congress not to vote on his request to authorize military action for now, as he wanted to give diplomacy a chance.

Senate majority leader Harry Reid had already postponed a test vote on an authorization of military force in Syria that had been expected to take place on Wednesday.

And given fast-sliding support for military action among lawmakers, and the American public at large, it is now uncertain when legislative action might take place.

Signs that there will be no US air strikes in the short term were also bolstered when Obama gave an assurance that there would be no military force used until United Nations inspectors have delivered their report into what happened on August 21.

The immediate political reaction to Obama's speech broke on expected lines -- and it was unclear if he had changed the minds of many lawmakers.