After Obama exited a limousine Thursday, he and Putin smiled, shook hands and chatted briefly before Obama entered the castle housing the annual, two-day summit.
While the summit is supposed to focus on the global economy, debate over possible military strikes in Syria in response to President Bashar Assad's apparent use of chemical weapons against his citizens could overshadow the conference.
The summit pits Obama, who wants to conduct limited military strikes, against Putin, whose country has stood by its long-time ally Assad. Sandwiched between are the views of the leaders of the 18 other G20 members, CNN said.
Calls for intervention in Syria heated up after U.S. officials said they had evidence of an alleged chemical weapons attack in rebel strongholds in Damascus suburbs last month that left about 1,400 people dead. A resolution that would grant Obama limited authority to conduct the military strike has been taken up by Congress.
During a media briefing aboard Air Force One traveling from Sweden, where Obama wrapped up his first visit, to St. Petersburg, Deputy National Security Adviser Ben Rhodes said no formal meeting between Obama and Putin was scheduled.
"However, it's always the case at these summits that leaders end up sitting next to each other; they end up having side conversations," Rhodes said. "So I certainly anticipate the president will have interactions with President Putin even as we don't have a formal meeting scheduled."
Rhodes said G20 sessions would be focused on the global economy while discussions about Syria could occur on the sidelines.
"I think that the past practice at these summits is you do end up having discussions on the margins of the meeting about other global security issues," Rhodes said. "We would not anticipate every member of the G20 agreeing about the way forward in Syria, particularly given the Russian position over many, many months now in terms of resisting efforts to hold the Assad regime accountable."
Obama, however, will have a chance to speak with U.S. allies and partners to "explain our current thinking on Syria" during the summit.
"And I think we'll continue to work with those countries to see what type of political and diplomatic support they may express for our efforts to hold the Syrian regime accountable," Rhodes said, later adding, "I think, similarly, the president will talk to allies and partners about their ability to express support for the notion that an international norm that the international community has spent many, many years reinforcing must be upheld in Syria. And I think we've found common views on that position among some of our key allies and we'll continue to discuss that with them here."
Soon after his arrival, Obama had a bilateral meeting with Japanese Prime Minister Shinzu Abe.
Before the meeting, Abe said through a translator that it was "extremely meaningful" for Obama to discuss Syria.
"I certainly look forward to continuously and closely working with you to improve the situation on the ground," Abe said.
Obama noted that the U.S.-Japan relationship was a "cornerstone of peace around the world."
He said the economy, jobs and growth would be the primary discussion points of the G20, and praised Abe for "very bold steps to boost growth and jobs and demand in Japan."
Obama also said the two leaders would maintain their discussions on security issues, including "our continued concerns about the nuclearization of the Korea peninsula and the importance of North Korea abiding by international law."
After the meeting, Rhodes, stressing he didn't speak for the Japanese, said "the U.S. and Japan were in agreement that there needs to be a response" to Syria and that the international norm prohibiting use of chemical weapons must be upheld.
"In the spirit of our alliance" the leaders expressed confidence that they could come to some "shared position" on Syria, he said.
Obama also met with Brazilian President Dilma Roussef between the first G-20 plenary session and dinner.
The summit dinner gathering of world leaders started late, with Obama arriving at Peterhof Palace, once the opulent summer home of Russia's tsars, about 1 hour, 45 minutes behind schedule. Obama walked down a long path to the palace by himself.
The evening's entertainment included what appeared to be chamber musicians and a large group of men and women in period costume, including powered wigs.
Obama spent 4 hours at the dinner -- which included a round of red-and-white fireworks at the conclusion -- returning to the villa where he's staying shortly after 2 a.m. Friday local time.