Obama under pressure to halt attack on Syria
After exchanging a stiff handshake and stern looks, the two men offered fixed smiles for the cameras before entering the Konstantinovsky Palace.
They were seated well away from each other in the summit room and did not speak directly during opening talks about the world economy, diplomats said.
Some leaders arriving in St Petersburg pointedly sided with Russia in its opposition to US-led military action.
Intervention would damage the global economy by pushing up oil prices, China said. Although Syria is not a significant oil exporter, the prospect of conflict in the Middle East usually has a significant effect on the market.
Brazil, India and South Africa were also worried that intervention would do economic harm, according to Mr Putin's officials.
While the British and French governments have backed the principle of military action to punish the Assad regime's alleged use of chemical weapons, the European Union as a whole is opposed, its officials said.
"There is no military solution to the Syrian conflict," he said.
"Only a political solution can end the terrible bloodshed, grave violations of human rights and the far-reaching destruction of Syria." But in New York, Dublin-born Samantha Power, the US ambassador to the United Nations, accused Russia of placing the future of the UN system in jeopardy by refusing to allow it to act against Syria.
"Russia continues to hold the council hostage and shirk its international responsibilities," she said.
Describing Moscow as "the patron of a regime that would brazenly stage the world's largest chemical attack in a quarter-century", Ms Power said its vetoing of intervention brought the UN into disrepute.
Separately, Pope Francis urged the Group of 20 leaders to abandon the "futile pursuit" of a military solution in Syria as the Vatican laid out its case for a negotiated settlement that guarantees rights for all Syrians, including minority Christians.
In a letter to the G20 host, Russian President Vladimir Putin, Pope Francis lamented that "one-sided interests" had prevailed in Syria, preventing a diplomatic end to the conflict and allowing the continued "senseless massacre" of innocents. "To the leaders present, to each and every one, 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," Pope Francis wrote as the G20 meeting got under way in St Petersburg.
Pope Francis will host a peace vigil in St Peter's Square on Saturday, a test of whether his immense popular appeal will translate into popular support for his peace message. It's the first time any such peace rally has been held at the Vatican, though Holy See officials have stressed it's a religious event, not a political protest.
British Prime Minister David Cameron had led international calls for a military response to the alleged chemical attack, but last week failed to win Commons backing for a British role in any military strikes.
He said: "I take full and personal responsibility for the decision to recall Parliament, for the decision to take a strong and principled stand against the gassing of children in Syria, and I take full responsibility for putting forward as generous a motion I could, to bring as many people with me as I could."
By James Kirkup in St Petersburg
(© Daily Telegraph, London)