Courtesy ABC News

President Obama arrived here today for the G20 summit, but all eyes will be on his showdown with Russian President Vladimir Putin over Syria.

The conflict in Syria is not actually on the two-day summit's agenda and the two leaders have no plans to meet one on one, but the subject of Syria will be unavoidable.

The Kremlin strongly opposes Obama's plan to intervene militarily in Syria, suggesting it is only a ploy to topple Syrian President Bashar al-Assad. Putin also believes any Western intervention will spark a wider conflict in the region. He has demanded that the United States receive authorization from the United Nations Security Council before taking action, something that Russia would surely block.

"Any other pretext or method which might be used to justify the use of force against an independent sovereign state is inadmissible and can only be interpreted as an aggression," President Putin said in an a joint interview with the Associated Press and a Russian state-owned television channel this week.

Obama, meanwhile, has sought to rally the world around his call to hold Assad accountable for his alleged use of chemical weapons. The United States has presented evidence it says shows Assad's forces used chemical weapons in an Aug. 21 attack on a Damascus suburb that the White House says killed over 1,400 people including hundreds of children.

"I didn't set a red line. The world set a red line. The world set a red line when government's representing 98 percent of the world's populations said the use of chemical weapons are abhorrent," Obama said during a visit to Stockholm, Sweden, on Wednesday.

In a sign that other parties are just as intently focused on Syria at this week's summit, which usually deals with only economic and trade issues, the United Nations' special envoy for Syria, Lakhdar Brahimi, is now expected to attend the G20 as part of Secretary General Ban Ki-Moon's delegation. Brahimi has been invited to attend a meeting of G20 foreign ministers on Friday morning.

Even before the debate over Syria, the meeting between Obama and Putin was already going to be awkward. The two were scheduled to hold a bilateral summit meeting this week, but Obama backed out last month after Russia granted intelligence leaker Edward Snowden asylum. American officials say the Snowden situation was only the straw that broke the camel's back, citing lack of progress on any of the issues the White House hoped to discuss, including missile defense and arms control.

The tension between the two leaders was apparently not lost on the summit's organizers. According to Russian news reports, they plan to seat them far from each other during group meetings.

Even so, the two are sure to see each other and will likely shake hands. But their body language may be telling.

The last time the two presidents met, on the sidelines of June's G8 summit in Northern Ireland, the differences between the two men and the countries they lead were laid bare. At a joint press conference, Obama was uncharacteristically cold and tense. Putin was perhaps even more so than usual.

That gulf has only widened in the past month, most strikingly over Syria.

Russia has been reluctant to throw Assad, its longtime ally, under the bus. Moscow remains Syria's biggest foreign backer, providing the government with much of the arms and cash it needs to sustain the now two and a half year conflict. Russia has urged the United States to wait for the results of a U.N. investigation into the Aug. 21 attack before taking action.

Obama, however, made clear that he holds Russian intransigence accountable for the ongoing violence.

"I think that international action would be much more effective and ultimately we can end deaths much more rapidly if Russia takes a different approach to these problems," he said on Wednesday.

The White House has expressed confidence that Congress will ultimately vote to authorize the use of force in Syria, even as the debate rages within both parties.

Putin does not appear content to sit back and watch things happen. Earlier this week he backed a proposal for Russian lawmakers to lobby members of Congress ahead of the vote, urging them to vote no.

Read on ABC News