Russian President Vladimir Putin on Friday said he held talks on Syria with U.S. President Barack Obama on the sidelines of the G20 summit but confirmed the meeting did not end their differences on the conflict, as the American leader said he knew convincing the U.S. Congress to back military action against Damascus would be a "heavy lift."

"We spoke sitting down... it was a constructive, meaningful, cordial conversation. Each of us kept with our own opinion," Putin told reporters, saying the meeting lasted 20 to 30 minutes.

Russian President Vladimir Putin's top critic Alexei Navalny prepared Friday to rouse supporters at the final rally of his fierce campaign for Moscow mayor before facing a Kremlin-backed incumbent in polls this weekend.

In a warning shot to the authorities, the charismatic leader of the Russian protest movement threatened protests if officials rigged Sunday's vote.

Russia has nothing to add to the political debate on Syria in the United States, a White House official said on Friday, also signaling that Washington is ready to take punitive action against the Syrian regime without the U.N. Security Council's backing.

"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 on this issue," said Ben Rhodes, President Barack Obama's deputy national security adviser for strategic communications, after senior Russian lawmakers expressed a desire to visit Washington and persuade Congress not to approve military action against Syria.

Russia on Friday warned the United States against targeting Syria's chemical arsenal as Washington considered the use of force against President Bashar Assad's regime.

"With particular concern we perceive the fact that military infrastructure facilities securing the integrity and safety of Syria's chemical arsenal are among the possible targets for military strikes," the Russian Foreign Ministry said in a statement.

Russian President Vladimir Putin discussed the situation in Syria with British Prime Minister David Cameron early on Friday, Putin's spokesman said.

Cameron and Putin met on the sidelines of the G20 summit in Saint Petersburg after discussing the Syrian crisis over dinner with other world leaders and watching a cultural show that ended around 2:00 am (2200 GMT).

World leaders at the G20 summit on Friday failed to bridge their bitter divisions over U.S. plans for military action against the Syrian regime, with Washington signaling that it has given up on securing Russia's support at the UN on the crisis.

A dinner hosted by Russian President Vladimir Putin that ran on into the early hours of the morning failed to win a breakthrough on how to halt a conflict in Syria that has claimed more than 100,000 lives and which is now in its third year.

NATO chief Anders Fogh Rasmussen sharply regretted international divisions over Syria on Thursday, warning that such splits risked sending a message of encouragement to dictators across the globe to use prohibited deadly weapons.

"I strongly regret that division within the international community," Rasmussen told Agence France Presse on the sidelines of an EU defense ministers' meeting taking place in Lithuania as the United States and Russia sought to overcome divisions over a U.S.-led push for military action against the Damascus regime.

The United States on Thursday accused Russia of holding the U.N. Security Council "hostage" over the Syria chemical weapons crisis.

With the White House pushing Congress to approve military strikes on Syria, U.S. ambassador to the United Nations Samantha Power said she could see no way to seek Security Council approval for action against President Bashar Assad because of Russia's blocking.

With a macho handshake, wide smiles for the cameras and a spot of small talk about the late summer weather, President Barack Obama and Russian counterpart Vladimir Putin sought Thursday to mask months of tensions that gave rise to talk of a bitter personal animosity.

The squirmingly awkward pictures from their last encounter at the G8 in June and Obama's cancellation of a bilateral summit in Moscow had created expectations of another tense encounter.

Pope Francis on Thursday appealed to leaders gathering for a G20 summit in Russia to find a peaceful solution to the Syria crisis, warning against "futile pursuit of a military solution."

"To the leaders present... I make a heartfelt appeal for them to help find ways to overcome the conflicting positions and to lay aside the futile pursuit of a military solution," Francis wrote in a letter to Russian President Vladimir Putin who is hosting the G20 summit.