The Group of 20 comprises finance ministers and central bank governors from 20 major economies: 19 countries plus the European Union, which is represented by the president of the European Council and by the European Central Bank.
Syria set to overshadow G20 summit in St Petersburg
President Putin offers Barack Obama a smile and a handshake, but there will be no one-on-one discussions between the leaders
Associated Press in St Petersburg, Russia
President Barack Obama and his Russian counterpart, Vladimir Putin, shook hands, smiled and made small talk about the scenery yesterday - a public exchange of pleasantries belying a tense relationship that only seems to be getting worse.
Parsing the body language between Obama and Putin has become something of a geopolitical parlour game every time the two leaders meet. But there wasn't much to work with this time: their exchange lasted just 15 seconds.
Obama's black armoured limousine pulled up to the palace where Putin was waiting to greet each of the leaders. The US president was the only leader who used his own official vehicle for the arrival, opting not to use the summit-issued Mercedes the other 19 leaders used.
The two leaders, both smiling, greeted each other with a handshake. Obama gestured towards the palace and the blue sky, declaring the location "beautiful".
Obama and Putin may talk again on the sidelines of the summit, but any discussion would be private.
Obama met Japanese Prime Minister Shinzo Abe on the sidelines and the US president said he and Abe shared the view that chemical weapons use in Syria was a violation of international law that must be addressed. Obama is also scheduled to meet French President Francois Hollande and Chinese President Xi Jinping today.
Still struggling to persuade dubious lawmakers at home on Syria, Obama in Russia will seek to win over world leaders reluctant to get drawn into yet another US-led sortie in a Mideast nation.
Although Syria wasn't formally on the agenda for the economy-focused summit, US officials were resigned to the fact that the civil war there would overwhelm any talks about global economics.
Putin announced that after requests by "some participants", the crisis in Syria would after all be discussed over Thursday's dinner, something that would have gladdened Obama.
In an ironic twist for Obama, the nation hosting the summit is also the nation most forcefully obstructing Obama's path to an international consensus. Russia has provided critical military and financial backing for Assad and has leveraged its veto power in the UN Security Council to keep a resolution condemning Syria from getting off the ground.
While insisting Obama has yet to prove his case, Putin appeared to temper his rhetoric slightly in a pre-summit interview, saying he wouldn't rule out backing a UN resolution if it can be proved Assad gassed his own people with chemical weapons.
He also played down any personal tensions with Obama.
"President Obama hasn't been elected by the American people in order to be pleasant to Russia," Putin said.
"And your humble servant hasn't been elected by the people of Russia to be pleasant to someone either."
Such an admission revealed a remarkable lowering of the bar from the summer of 2009, when Obama, on his last visit to Russia, trumpeted a "reset" in relations between the former foes.
"This will not be easy," Obama said in Moscow. "It's hard to change habits that have been ingrained in our governments and our bureaucracies for decades."
Indeed, it hasn't been easy. The crisis in Syria joins a long list of contentious issues that have made co-operation between the countries a trying endeavour, even though Obama points to successes early in his presidency on nuclear stockpile reduction and trading regulations.
Obama will call attention to one area of disagreement - gay rights - when he meets today in St Petersburg with lesbian, gay, bisexual and transgender activists.
Additional reporting by Reuters