ST. PETERSBURG, Russia » President Barack Obama acknowledged deep divisions at home and abroad today over his call for military action in Syria — and conceded the possibility he’ll fail to sway the American public. He refused to say whether he would act without passage of congressional authorization for a strike in response to chemical weapons use.

Setting the stage for an intense week of lobbying in Washington over the strike resolution, Obama said he planned to make his case to the American people in an address Tuesday night.

Obama laid out in new detail his reasoning for seeking congressional approval, saying it was because the use of chemical weapons in Syria didn’t pose an imminent threat to the United States or its allies — situations in which he said he would have responded immediately. But he said the use of weapons of mass destruction is a long-term threat to the United States and the world, and the U.S. has the ability to respond with air strikes without the risk of putting troops on the ground.

"It’s conceivable at the end of the day I don’t persuade a majority of the American people that it’s the right thing to do," Obama acknowledged. "And then each member of Congress is going to have to decide."

Obama, speaking at the end of a two-day Group of 20 economic summit, earlier held a surprise meeting with Russian President Vladimir Putin, a chief opponent of U.S. military action. Both Obama and Putin said that while they still disagreed, the meeting was constructive.

Obama, in his news conference, seemed to be feeling the burden of the challenge he faces in persuading the American public, the international community and Congress to back military action. But he expressed confidence the American people and lawmakers, weary after long-running wars in Iraq and Afghanistan, would listen.

"I trust my constituents want me to offer my best judgment. That’s why they elected me. That’s why they re-elected me," he said.

Ten members of the Group of 20 joined the United States in a joint statement accusing the Syrian government of carrying out a chemical weapons attack on civilians last month and calling for a strong international response against the government of Syrian President Bashar Assad. The statement stopped short of explicitly calling for military action against Syria, but administration officials argued that it amounted to support of Obama’s move toward targeted military strikes.

The countries signing the statement with the U.S. were Australia, Canada, France, Italy, Japan, South Korea, Saudi Arabia, Spain, Turkey and the United Kingdom.

Seeking to rally support back in Washington, the administration planned another classified briefing for all lawmakers next Monday evening with Secretary of State John Kerry, Defense Secretary Chuck Hagel, Director of National Intelligence James Clapper, Joint Chiefs of Staff Chairman Gen. Marin Dempsey and White House National Security Adviser Susan Rice. And White House Chief of Staff Denis McDonough planned to attend the closed-door Democratic caucus meeting Tuesday morning, according to a congressional aide.

Senate Majority Leader Harry Reid today formally introduced the resolution, which would authorize the "limited and specified use" of the U.S. armed forces against Syria for 90 days while prohibiting American ground troops from combat. Lawmakers return from their five-week recess on Monday and will begin to debate, with a Senate vote to move ahead on the resolution expected Wednesday.

"I think we’re going to get 60 votes. It’s a work in progress," Reid told reporters.

Although surveys showed a significant number of House Republicans and Democrats opposed to military action or leaning against it, officials in the leadership insisted it was premature to say the resolution could not be approved. At this stage, just a third of the House and Senate have participated in classified briefings and Obama is still reaching out to lawmakers.

Still, final passage rests on significant votes from House Republicans and Democrats, and the administration is struggling to reach those numbers. A spokesman for Republican House Speaker John Boehner, who is backing authorization, said Obama needs to make a convincing case to the public Tuesday night.

"Members of Congress represent the views of their constituents, and only a president can convince the public that military action is required. We only hope this isn’t coming too late to make the difference," spokesman Brendan Buck said.

Obama said he and other leaders at the summit had had a "full airing of views on the issue" during a three-hour dinner Thursday night. He said the leaders were unanimous in believing that chemical weapons were used in Syria and that international norms against that use must be maintained. He said division comes over whether action must come through the United Nations.

Putin said that the U.S. push for military action has been supported only by Turkey, Canada, Saudi Arabia and France. Putin said that German Chancellor Angela Merkel has taken a "very cautious attitude."

He said that many others along with Russia and China voiced their opposition to the military action, including India, Indonesia, Argentina, Brazil, South Africa and Italy.

"The use of force against a sovereign nation is only possible as self-defense — and Syria hasn’t attacked the United States — and on approval of the U.N. Security Council," Putin said. "Those who do otherwise place themselves outside the law."

British Prime Minister David Cameron supports military intervention, but the British Parliament opposed him and so he says his country won’t be part of any action. Still, he argued that an international response is needed and shouldn’t be held up by the U.N. Security Council. "If we say that there can only be a response if the U.N. Security Council votes for it, we are in fact contracting out our foreign policies, our morality, to the potential of a Russian veto. I think it’s a very misguided approach," he said.

Russia today warned the United States and its allies against striking any chemical weapon storage facilities in Syria. The Russian foreign ministry said such targeting could release toxic chemicals and give militants or terrorist access to chemical weapons.

"This is a step toward proliferation of chemical weapons not only across the Syrian territory but beyond its borders," the Russian statement said.

Meanwhile, the Kremlin said Russia was boosting its naval presence in the Mediterranean Sea, moving in warships into the area and stoking fears about a larger international conflict if the United States orders airstrikes.

Illustrating the risks associated with a strike, the State Department ordered nonessential U.S. diplomats to leave Lebanon, a step under consideration since last week when Obama said he was contemplating military action against the Syrian government.