Vladimir Putin stood alone on a white carpet in front of the sun-drenched Constantine palace, pleasantly welcoming one smiling leader after another as they emerged like clockwork in a steady parade of black Mercedes limos.

But when President Barack Obama arrived last in his large black Cadillac, the Russian president was on the receiving end of a stern and somber look from his U.S. counterpart. Eventually they looked at the wall of photographers and smiled for the cameras, but it was clear the tension between the two men is far from exaggerated.

Observers of U.S.-Russian relations have stated recently that the presidential ties between the two nations are at their most tenuous in decades. Yet as they gather for dinner Thursday with their G20 colleagues, they will be the most closely watched as the world debates a potential military mission in Syria.

Story continues below advertisement

Dmitry Peskov, press secretary to Russian President Vladimir Putin, said he expects there will be some discussion between the two leaders.

"As they are attending the same event for about two days, with quite big confidence I can say contact can take place," he said. "But specially-planned meetings are not envisioned, but we hope that they will have a chance to make a quick exchange of opinions."

Mr. Putin – the most vocal critic of Mr. Obama's intentions for a military strike in Syria – gained support for his argument this week as China joined Russia in opposing a military strike.

Pope Francis, meanwhile, urged the leaders present to find ways to overcome their conflicting positions and abandon the "futile pursuit" of a military option, according to the Associated Press.

The G20 leaders' summit is traditionally focused on economic matters, but the situation in Syria is posed to overwhelm the summit agenda. Foreign affairs ministers were invited at the last minute to hold meetings on the sidelines that are directly focused on Syria, including the humanitarian impact of more than two years of civil strife.

Canada is strongly supportive of the U.S. position on Syria and has been a vocal critic of Mr. Putin's rhetoric in support of Syria. Nonetheless, Prime Minister Stephen Harper and Mr. Putin had a friendly exchange that was similar to the arrivals of other leaders.

In addition to Mr. Obama, French President François Hollande also appeared to give Mr. Putin a cool greeting. So far France is the only European state that has pledged to join the U.S. in a military strike on Syria.

Story continues below advertisement

London's Telegraph newspaper reported that Mr. Obama will meet Mr. Hollande, but not British Prime Minister David Cameron, while in St. Petersburg. The report suggested the timing could be as a snub of Mr. Cameron for failing to secure political support in the U.K. for military action.