US Edition President Barack Obama and his Russian counterpart could not even agree on the factual point of whether a majority of G20 members supported or opposed military action.
Mr Obama said that most of the 19 countries represented at St Petersburg had backed the US position – and 11 duly signed a statement urging a "strong international response" to the poison gas attacks in Damascus.
But Mr Putin disputed this, pointing out that although David Cameron might have signed the statement, Parliament's vote against British military action showed that the Prime Minister did not speak for his country.
Ending the summit, Mr Putin said that world opinion was firmly against US-led intervention, adding that Russia would take Syria's side. "Will we help Syria? We will," he said. "We are already helping, we send arms."
Russia has been Syria's biggest arms supplier, signing a contract to deliver the S300 air defence system. Some components of this advanced defensive shield are understood to have been delivered, although whether Russia has supplied everything is unclear.
In recent weeks, the Russian navy has also sent three warships to the eastern Mediterranean, near the Syrian coast, where it maintains a base at the port of Tartous. Russian reports suggest that a fourth vessel, carrying a "special cargo", is now heading to the area.
When a journalist suggested that the G20 was evenly split over military action, Mr Putin replied: "You said views divided 50-50, that is not quite right". He listed only the US, Turkey, Canada, Saudi Arabia and France as countries supporting an intervention.
Mr Putin added that Angela Merkel, the German Chancellor, had remained "careful" on this issue. As for Mr Cameron, the Russian president accepted that he supported a military strike, but added that the Prime Minister does not represent the "will of the people" because of parliament's position.
Meanwhile, China, India, Indonesia, Argentina, Brazil, South Africa and Italy were all "against military action," he said. Even in countries where the government happened to support action, "the majority of the population is on our side," added Mr Putin.
"Using force against a sovereign state can only be done in self defence, and Syria is not attacking the United States," he said. "As one participant said yesterday, those who do something different are placing themselves outside the law."
Mr Putin and Mr Obama did not have a formal bilateral meeting during this summit – and the American president unilaterally cancelled a planned meeting in Moscow. But the two men had an informal conversation on the margins of the event.
"It was a constructive, meaningful, cordial conversation," said Mr Putin. He added that it lasted 20 to 30 minutes and "each of us kept with our own opinion".
Mr Putin said: "There is dialogue: we hear each other and understand the arguments. He [Mr Obama] disagrees with my arguments, I disagree with his arguments, but we do hear, and we try to analyse."
In the end, the summit's divisions over Syria prevented agreement on a joint statement signed by all of 19 countries – the 20th is the European Union. Instead, one was signed by the US and Britain as well as Australia, Canada, France, Italy, Japan, South Korea, Saudi Arabia, Spain and Turkey.
Referring to the gas attacks in Damascus on Aug 21, these countries said: "We call for a strong international response to this grave violation of the world's rules and conscience that will send a clear message that this kind of atrocity can never be repeated. Those who perpetrated these crimes must be held accountable."