The U.S. President also held a surprise meeting with his Russian counterpart Vladimir Putin, a chief opponent of U.S. military action.

President Barack Obama claimed a growing recognition among foreign leaders on Friday that “the world cannot stand idly by” in the face of chemical weapons use in Syria, and said he plans to make his case to the American people in an address on Tuesday night.

With Congress showing signs of reluctance to back a resolution authorising military strikes, Mr. Obama refused to say whether he would act if he fails to win that approval.

“It would be a mistake for me to jump the gun and speculate because right now I’m working to get as much support as possible out of Congress,” he said.

Mr. Obama told reporters at the end of a two-day Group of 20 economic summit that he and other leaders had had a “full airing of views on the issue.” He said many foreign nations will be issuing statements on their positions, but he didn’t say whether any specifically had joined France in supporting his move toward U.S. military strikes.

Mr. Obama also held a surprise meeting with Russian President Vladimir Putin, a chief opponent of U.S. military action. Mr. Putin, a staunch ally of Syrian President Bashar Assad, said this discussion with Mr. Obama focussed on Syria during the 20 to 30 minutes and that, while they disagreed, the meeting was “substantial and constructive.”

Mr. Obama said he had a “candid and constructive conversation” with Mr. Putin, even if they still disagreed on how to respond to the chemical weapons use in Syria. He said they agreed the underlying conflict can only be resolved through a political transition. Mr. Obama said he thinks it is important that he and Mr. Putin work together to urge all sides in the conflict to try to resolve it.

The meeting came on Friday as Mr. Obama sought to build international backing for military action. But three days after he left Washington, it’s unclear whether the global coalition the president has been seeking is any closer to becoming a reality.

Putting up stiff resistance to Mr. Obama’s appeals, Russia on Friday 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.

Moreover, China remained a firm no. The European Union is skeptical about whether any military action can be effective. Even Pope Francis weighed in, urging leaders gathered here to abandon what he called a “futile mission.”

Still, Mr. Obama was undeterred. He and French President Francois Hollande, the U.S.’s strongest ally on Syria and a vocal advocate for a military intervention, met on the sidelines of the summit about attracting European support for action. “It’s clear that there are many countries that agree with us that international norms must be upheld,” Obama said.

“To do nothing would mean impunity,” Hollande said. “We must take our responsibility” and act.

As the President pressed his case on the world stage, he was dispatching his U.N. ambassador, Samantha Power, to a Washington think tank to argue that the global community cannot afford the precedent of letting chemical weapons use go unpunished.

Illustrating the risks associated with a strike, however, the State Department on Friday ordered nonessential U.S. diplomats to leave Lebanon, a step under consideration since Obama said he was contemplating military action against the Syrian regime last week. The travel warning said it had instructed nonessential staffers to leave Beirut and urged private American citizens to depart Lebanon.

Yet even as Mr. Obama sought the global buy-in that could legitimise a potential strike, his aides were careful to temper expectations that the world community could speak with one voice. Mr. Obama’s Deputy National Security Adviser, Ben Rhodes, said the President wasn’t asking his peers to pledge their own militaries to a U.S.-led strike, but simply to say they agree a military response is warranted.

“We don’t expect every country here to agree with that position,” Mr. Rhodes said Friday at the Group of 20 economic summit, where Mr. Obama was huddling with foreign leaders.

Standing on Russian soil, Mr. Rhodes suggested the U.S. had given up hope that Russia a stalwart Syria ally could be coerced into changing its position. “We don’t expect to have Russian cooperation,” he said.

A key status update was to come Friday when Mr. Obama, his diplomatic dexterity pushed to the max, will be quizzed by reporters in the waning hours of the summit.

A jobs-and-growth agenda awaiting world leaders gathering at the ornate Constantine Palace quickly gave way to intense posturing over Syria at least on the surface. The leaders served up Syria as dinner conversation on Thursday at the suggestion of the summit’s host, Mr. Putin. The Russian leader has steadfastly backed Mr. Assad and disputes claims that the Syrian regime was behind chemical attacks that the U.S. says killed more than 1,400 Syrians. Other estimates are lower.

Syria dominated the nearly three-hour meal, with leaders condemning the use of chemical weapons but reaching no consensus about the proper response, said a French official in St. Petersburg. Many leaders at the dinner remained in doubt about whether Mr. Assad’s regime was behind the attack, said the official, who was not authorized to be publicly named according to presidential policy.

So too was the Syrian crisis a prevailing theme in Obama’s individual meetings with world leaders on the sidelines of the summit in this Russian port city.

The White House said Obama conferred on Syria Thursday evening with Turkish Prime Minister Recep Tayyip Erdogan, a strong supporter of airstrikes against the nation on its southern border. Syria also came up on Friday as Mr. Obama met with Chinese President Xi Jinping, whose government has warned vigorously against the use of force.

Before his scheduled return to Washington late Friday, Mr. Obama also planned to meet with Russian lesbian, gay, bisexual and transgender activists, calling attention to another area of disagreement with Moscow.

A fleeting interaction between Mr. Obama and Mr. Putin became the high-drama moment of the summit, underscoring the laboured state of relations between the two leaders. The eyes of the world watching, the Russian and the American were all smiles on Thursday as they made small talk in front of news cameras for a few seconds as Mr. Obama arrived at the summit.

But the welcoming handshake may have been where the pleasantries ended. In other venues, the two nations were repeatedly bumping heads.

Russia’s Foreign Ministry, in a statement on Friday, said a U.S.-led strike would mark “a new dangerous turn” in the crisis, risking the release of chemical weapons or their possession by terrorists. And the head of the foreign affairs panel in Russia’s lower house of parliament, Alexei Pushkov, blasted Mr. Obama on Twitter as having “completely transformed into a president of war.”

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.

Even at home, there was far from a consensus that an American strike on Syria was the best course of action. Awaiting Mr. Obama upon his return was an equally fractious debate in Congress over whether to authorize the limited military action he was proposing.

Pulling out all the stops, Mr. Obama was working the phones from Europe and appealing for support from leery legislators, Democratic and Republican alike. And he called off a planned trip to California next week, opting to stay in Washington to keep up the pressure on Congress to say yes.

As top national security officials continued to brief Congress on the accusation against Mr. Assad and the proposed response, a measure authorising Mr. Obama to act was advancing tenuously through the Senate, winning approval from a foreign relations panel Wednesday and heading to the Senate floor. The measure’s prospects were less certain in the Republican-controlled House.

In an unusual turn of events, Senate Majority Leader Harry Reid and House Speaker John Boehner both said that a delegation of Russian lawmakers had sought to meet with them to discuss Syria. Both leaders, who are supporting Mr. Obama’s call for a strike, turned down that invitation, aides said.

“I don’t know that the Russians have anything to add to the debate in the United States, given that we know where Russia stands,” said Mr. Rhodes, the Obama aide.

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