Western bombs are unlikely to fall on Syrian government targets during this gathering. The US and French presidents are readying possible military strikes over what they say was a chemical weapons attack by Syrian President Bashar Assad's army, but both are waiting for the US Congress to weigh in first.
In the meantime, Obama and Hollande came under pressure and criticism from opponents of intervention, as China and EU leaders urged restraint. The UN's Ban is pressing for diplomatic action.
Putin, on his own turf and looking strong in the face of Western hesitancy to tangle militarily with the Russia-backed Assad, told The Associated Press this week that any one-sided intervention would be rash. But he said he doesn't exclude supporting UN action - if it's proven that the Syrian government used poison gas on its own people. And China is among those warning at this summit that oil price volatility resulting from an international Syria war could threaten global economic recovery.
US-RUSSIA BODY LANGUAGE
Even without Syria, Obama and Putin had plenty to disagree about.
Obama snubbed the Russian leader, cancelling a one-on-one meeting over lack of progress on other issues too - including Russia's harbouring of Edward Snowden, the National Security Agency contractor who exposed US surveillance of emails and phone calls of Americans and foreigners. The US also strongly opposes arrests of political opponents and a new law against gay "propaganda".
Body language may be key to understanding where the US-Russia relationship is going. The two leaders shook hands as the summit opened, but Obama's face was stern as he watched Putin open the G20 talks. Will they relax and make small talk over dinner at Peter the Great's resplendent Peterhof Palace? Will other leaders take sides?
TAXING MULTINATIONALS
The goal of some leaders at this G20 is to get major cross-border companies such as Google and Starbucks paying more taxes instead of using loopholes and tax havens.
Laudable to the general public, it's complicated both practically and politically. It would require cracking down on well-connected companies registered in Delaware or the Virgin Islands, for example.
But if any forum can tackle this, it's the G20, with all the major government decision-makers at the table. Some leaders also want to rein in so-called shadow banking and regulate hedge funds more.
FED UP WITH THE FED
The developing economies whose vigorous growth helped the world economy survive the financial market meltdown five years ago are now starting to falter. And they're placing part of the blame on the US Federal Reserve's expected moves to wind down stimulus measures. China and Russia started off the summit by warning the US to consider international fallout as they set monetary policy.
That expectation has pushed up long-term US interest rates, which has in turn led investors to pull out of developing countries and invest in U.S. assets instead. The leaders of Russia and Brazil and others may appeal for the US to coordinate with other governments when it changes financial policy.
OLYMPIC PRESSURE
With Russia set to host the Winter Olympics in Sochi in five months, this summit is the place for other leaders to pressure Putin to open up his country and himself to criticism, opposition and public debate.
Activists want pressure against Russia's gay propaganda law, a law banning adoptions by Americans, and legal cases targeting Putin opponents. The Russian leader, for his part, wants global recognition, and revenue, from these games.
Here's a look at key Syria developments around the world amid heightened tensions over potential military action:
RUSSIA:
Putin's spokesman, Dmitry Peskov, said the US should wait for the report of UN inspectors who investigated a chemical attack in Syria before intervening militarily, adding that Washington's evidence of the Syrian regime's involvement isn't strong enough. He insisted the UN Security Council is the sole body that can authorize the use of force. The Kremlin's chief of staff said Russia has been sending warships to the Mediterranean Sea for possible evacuation of Russian citizens from Syria. Sergei Ivanov was quoted by Russian news agencies as saying Russia boosted its naval "primarily" to organise a possible evacuation of Russians from Syria.
SYRIA:
Syrian government troops battled al-Qaeda-linked rebels over a regime-held Christian village in western Syria for the second day. Residents of Maaloula said the militants entered the village late Wednesday. Rami Abdul-Rahman, the director of the Britain-based Observatory for Human Rights, said the fighters included members of the of al-Qaeda affiliated Jabhat al-Nusra group.
UNITED STATES:
Obama pressed sceptical lawmakers in phone calls from St. Petersburg to give him the authority to use US military force against Syria while the administration. Obama's advisers were pressing Congress in closed-door meetings for authorisation of a military strike on Syria.
UNITED NATIONS:
From St. Petersburg, UN Secretary-General Ban Ki-moon said the situation in Syria has "no military solution. There is only a political solution which can bring peace and end this bloodshed right now."
EUROPEAN UNION:
European Union President Herman Van Rompuy said urged UN investigators to release information as soon as possible about a chemical weapons attack in Syria so the international community can decide how to respond. Rompuy said in St. Petersburg that the attack "was a blatant violation of international law and a crime against humanity". He said it is too early for a military response.
AUSTRIA:
Russian Foreign Ministry spokesman, Alexander Lukashevich, warned a US strike on Syria's atomic facilities might result in a nuclear catastrophe and urged the UN's nuclear agency to present a risk analysis of such a scenario. Gill Tudor, spokeswoman for the Vienna-based International Atomic Energy Agency, said the IAEA is ready to "consider the questions raised" by Lukashevich if it receives a formal request from Moscow.
GERMANY:
German Chancellor Angela Merkel said she doubts world leaders can agree on what to do about Syria's civil war despite frenzied diplomatic efforts because of disagreement over who was responsible for the poison gas attack. She said: "I do not believe yet that we will reach a joint position."
VATICAN:
Pope Francis urged world leaders to abandon the "futile pursuit" of a military solution in Syria and work instead for dialogue and negotiation to end the conflict. In a letter to Putin, the pope lamented that "one-sided interests" had prevailed in Syria. He said those interests have prevented a peaceful solution and allowed the continued "senseless massacre" of innocents.
CHINA:
China warned of global economic risks linked to a potential US-led military intervention in Syria's civil war. Chinese Vice Finance Minister Zhu Guangyao says such "military action would definitely have a negative impact on the global economy, especially on the oil price".
FRANCE:
An international aid group that supports doctors in war zones said one of its Syrian surgeons was killed in an attack the northern Syrian province of Aleppo. Dr Muhammad Abyad, 28, had been working in an Aleppo hospital run by the group also known as Medecins Sans Frontieres.
LITHUANIA:
NATO Secretary-General Anders Fogh Rasmussen said from Vilnius, Lithuania's capital, that any strike against Syria would not need NATO's command and control system because it would probably be ""a short, targeted, tailored military operation". NATO has already said it would defend Turkey in case the member state was attacked as part of the Syria crisis.
- AP
