U.S. President Barack Obama vowed to pursue a diplomatic initiative from Russia over Syria's chemical weapons on Wednesday but voiced skepticism about it and urged Americans to support his threat to use military force.

Obama said in a speech that a Russian offer to push Syrian President Bashar Assad to place chemical weapons under international control offered the possibility of heading off the type of limited military strike he is considering against Syria.

Speaking from the White House's East Room, Obama said U.S. and Russian officials would keep talking about the initiative and that he would discuss it with Russian President Vladimir Putin.

Meanwhile, he said, he has asked the U.S. Senate to put off a vote on his request for an authorization of military force to let the diplomacy play out. He set no timetables for action, but said any deal with Assad would require verification that he keep his word.

"It's too early to tell whether this offer will succeed. And any agreement must verify that the Assad regime keeps its commitments. 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."

However, the U.S. president said he has ordered the military to maintain its position in order to pressure Assad in case diplomacy fails. As for the danger of retaliation, Obama said he does not take Assad's threats lightly, but made it clear that Syria's army is no match for the U.S. and its allies.

"The Assad regime does not have the ability to seriously threaten our military… Neither Assad nor his allies have any interest in escalation that would lead to his demise, and our ally, Israel, can defend itself with overwhelming force, as well as the unshakable support of the United States of America." 

The Russian offer put the brakes on a vote in Congress over authorizing military force as lawmakers and the administration sought more time to assess Russia's proposal to put Syria's chemical weapons under international control.

Obama has faced stiff resistance in Congress to any military action and lawmakers on both sides of the issue were quick to seize on the Russian proposal as a possible way out, despite skepticism about its eventual success.

Obama used much of his speech to lay out the case against Syria, saying there was plenty of evidence showing that the Syrian government was behind an Aug. 21 chemical weapons attack that killed 1,429 people, including more than 400 children.

He argued that Syria should face consequences for using such weapons because much of the world has long since adopted a ban on chemical weapons and that if the civilized world does nothing to respond, it will only embolden U.S. adversaries.

"If we fail to act, the Assad regime will see no reason to stop using chemical weapons," said Obama. "As the ban against these weapons erodes, other tyrants will have no reason to think twice about acquiring poison gas and using them."

A group of Republican and Democratic U.S. senators began drafting a modified resolution on the use of military force that would give the United Nations time to take control of Syria's chemical weapons.

The Russian diplomatic initiative, which emerged after off-the-cuff remarks by Kerry on Monday alluding to such a deal, marked a sudden reversal following weeks in which the West seemed headed toward intervening into Syria's 2-1/2-year-old civil war.

While pledging to pursue a diplomatic outcome, Obama spent much of the speech outlining why he feels the United States has a global obligation to respond to the Syrian chemical weapons onslaught. He portrayed the August 21 attack in graphic terms.

"The images from this massacre are sickening: men, women, children lying in rows, killed by poison gas; others foaming at the mouth, gasping for breath; a father clutching his dead children, imploring them to get up and walk," he said. "In World War I, American G.I.s were among the many thousands killed by deadly gas in the trenches of Europe. In World War II, the Nazis used gas to inflict the horror of the Holocaust."

He said any U.S. military action would be limited, nothing like the Iraq and Afghanistan wars that he has spent much of his presidency winding down. No American troops will be on the ground in Syria if action is taken, Obama said.

"This would be a targeted strike to achieve a clear objective, deterring the use of chemical weapons and degrading Assad's capabilities," he said.

Any strike would still have an impact, he insisted, pushing back against some members of Congress who argue there is no point in doing a "pinprick" strike in Syria.

"Let me make something clear: The United States military doesn't do pinpricks. Even a limited strike will send a message to Assad that no other nation can deliver," he said.

Still, the overwhelming message from Obama was that he would like to avoid taking military action, a reflection of his own personal view toward diplomacy and a response to polls showing Americans are opposed.

Initial reaction to the speech from some undecided lawmakers showed skepticism about military intervention.

"America should bring the world together to condemn and penalize Syria for this action. Such an effort, however, is best pursued through international negotiation and diplomacy," Democratic Senator Jeff Merkley of Oregon said.