"I have ... asked the leaders of Congress to postpone a vote to authorize the use of force while we pursue this diplomatic path," Obama said in his prime-time speech Tuesday. "It's too early to tell whether this [diplomatic] offer would succeed."
The proposal, offered Monday by Foreign Minister Sergei Lavrov, calls on Syria to turn over its stockpile of chemical weapons to international control, destroy the weapons and sign on to the Organization for the Prohibition of Chemical Weapons.
Obama said he was sending Secretary of State John Kerry to meet Lavrov Thursday and would "continue my own discussions" with Russian President Vladimir Putin.
In his 15-minute speech from the East Room at the White House, Obama called Syria's use of chemical weapons a danger to American security and the security of its allies and said it violated the world's "sense of common humanity."
On Aug. 21, he said, "the world saw in gruesome detail the terrible nature of chemical weapons and why the overwhelming majority of humanity has declared them off-limits, a crime against humanity and a violation of the laws of war. ... Moreover, we know the Assad regime was responsible. "
Failure to respond would embolden Assad, other dictators and bad actors to seek out and use chemical weapons, among other things.
Earlier, the White House said the leaders of the United States, France and Britain Tuesday agreed to consider a proposal for Syria to give up its chemical weapons.
Obama had separate phone conversations with French President Francois Hollande and British Prime Minister David Cameron in which they agreed to "work closely together, and in consultation with Russia and China, to explore seriously the viability of the Russian proposal to put all Syrian chemical weapons and related materials fully under international control in order to ensure their verifiable and enforceable destruction," a statement released by the office of White House press secretary Jay Carney said.
"These efforts will begin today at the United Nations, and will include a discussion on elements of a potential U.N. Security Council Resolution," the statement said. "The leaders discussed their preference for a diplomatic resolution but stressed the importance of continuing to develop a full range of responses by the international community to the Assad regime's use of chemical weapons near Damascus on Aug. 21."
Meanwhile, several U.S. senators worked on an alternative congressional resolution that would give the United Nations time to gain control of the Syrian chemical weapons stockpile, The New York Times reported. The Senate leadership has put off a vote on beginning debate on a resolution put forth by Obama.
The newspaper said Cameron told a parliamentary committee in London the proposal floated by Syria and Russia "is a serious proposal then we should act accordingly and I think a U.N. Security Council Resolution is a good idea."
NBC News reported Senate Minority Leader Mitch McConnell of Kentucky said he opposes Obama's call for a U.S. military strike against Syria for allegedly using chemical weapons in an Aug. 21 attack in which about 1,400 Syrians died.
"I will be voting against this resolution," he said on the Senate floor. "A vital national security risk is clearly not at play.
"The president's proposal seems fundamentally flawed, since if it's too narrow, it may not deter [Syrian President Bashar] Assad's use of chemical weapons, but if it's too broad, it risks jeopardizing the security of these same stockpiles, essentially putting them into the hands of extremists."
Syria's agreement to put its chemical weapons under international control is a result of U.S. pressure, Carney said Tuesday.
Speaking on MSNBC, the press secretary said Assad's acceptance of the plan was the result of "the credible threat of U.S. military action."
Carney said the proposal from Russian Foreign Minister Sergei Lavrov was "a potentially positive development" but that the president will continue with his plan to address the American people Tuesday night to press his case "that the international community, in this case led by the United States, cannot stand by and allow that international prohibition against the use of chemical weapons to unravel before our eyes."
Assad's agreement to the Russian proposal, Carney said, "is the byproduct of the push for action that the president has led."
"Before this morning, the Syrian government had never even acknowledged they possessed chemical weapons. Now they have," he said.
Because Obama does not see "an imminent threat" to national security, the president will press his case for Congress to "take action, vote and authorize the strike."
Noting the apparent public reluctance to get involved in another conflict in the Middle East, Carney said, "We need to get out there and make the case and make clear why this is important."
Syrian Foreign Minister Walid al-Muallem told the speaker of the Russian Parliament that Lavrov's terms would "remove the grounds for American aggression," The Independent of London reported.
"We held a very fruitful round of talks with ... Lavrov yesterday [Monday], and he proposed an initiative relating to chemical weapons. And in the evening we agreed to the Russian initiative," Muallem said.
Syria's agreement came as France announced it would submit to the U.N. Security Council a "binding resolution with a short time frame" that would force Assad to cede stockpile to international control, destroy the stockpile and sign on to the Organization for the Prohibition of Chemical Weapons.
Obama has asked Congress for authority to conduct a limited military strike after evidence indicated Assad's troops gassed citizens during an attack on Damascus suburbs Aug. 21.
French Foreign Minister Laurent Fabius said the Syrian government must be forced to reveal the full extent of its chemical program immediately, and that France would proffer a proposal to the U.N. Security Council Tuesday.
"The Russian foreign minister made an offer," Fabius said, "[but] this cannot be used as a maneuver to divert us."
"That is why we have decided to take this initiative. France will put forward a resolution at the U.N. Security Council in this sense and the procedure starts today," he said.
Fabius stressed that all options "remain on the table," including a targeted missile strike.
Earlier Tuesday, Russia said it was working with Assad's government to draw up an "effective, clear, concrete" plan for handing over Syria's chemical weapons.
Lavrov had told reporters the proposals would be circulated to other nations soon, stressing the fluidity of the situation and noting that conversations with Syrian officials were "being conducted literally at this minute."
"We hope to present this plan in the very near future, and will be prepared to finalize it and work it out with the involvement of the U.N. secretary-general, the Organization for the Prohibition of Chemical Weapons and members of the Security Council," Lavrov said.
The Lavrov plan won support from China, who, along with Russia, has resisted anti-Syria resolutions brought to the Security Council.
In Paris, Fabius said he expected a "nearly immediate" commitment from the Syria authorities and that Russia had information about Syria's chemical weapons stockpile.
The French proposal will call for Syria to allow OPCW inspectors to oversee the destruction of chemical weapons and will require that Syria become a member of the organization.
If there are any deviations, "extremely serious consequences" would be on the table, he said.
Lavrov's proposal came after U.S. Secretary of State John Kerry first raised the idea in an off-handed way that made clear the idea of Assad giving up Syria's chemical weapons stockpile was unlikely.
Lavrov said he discussed the proposal with U.S. officials before announcing it at a briefing Monday, the Times reported.
Obama and Russian President Vladimir Putin discussed the idea privately on the sidelines of last week's Group of 20 summit in St. Petersburg, and Lavrov discussed it with Kerry, the Times said.