WASHINGTON (AFP) - Sharp policy clashes over fugitive leaker Edward Snowden and Syria may be exacerbated by apparent personal animosity when President Barack Obama and Vladimir Putin come face-to-face in Russia this week.
Mr Obama will arrive in St Petersburg on Thursday for the G20 summit, after cancelling a one-on-one encounter with Mr Putin in Moscow, as a string of policy rows ranging from human rights to geopolitics rage between the Kremlin and the White House.
The testy personal relationship between Mr Putin and Mr Obama, and the diverging policy agendas, have put paid to the "reset" of relations which Mr Obama engineered with former Russian president Dmitry Medvedev early in his first term.
The G20 summit of developed and developing nations comes with many analysts diagnosing the state of US-Russia relations as at their worst since the end of the Cold War.
While US officials were infuriated by Russia's offer of asylum to Snowden, who leaked National Security Agency snooping secrets, they said Mr Obama cancelled the Moscow summit because of a lack of progress across a wide range of issues.
But it is largely Snowden, and Russia's support for its last major Middle Eastern ally Syria, that have brought the relationship to its current low.
Frigid personal ties between Mr Obama and Mr Putin were in evidence when the two held an awkward photo op when they met in Northern Ireland at the G8 summit in June.
Then a few weeks ago, Mr Obama declared that he did not have a bad personal relationship with Mr Putin, but then went on to mock his counterpart as "the bored kid at the back of the classroom." The New York Times reported that Mr Putin was infuriated by the comment.
"The good news is that this is not the Cuban Missile Crisis," said Andrew Kuchins, a Russia expert at the Center for Strategic and International Studies.
"But one thing is clear to me - that this is the worst personal relationship between US and Russian, perhaps even US and Soviet, leaders in history," Mr Kuchins said.
"I really think these two guys, Mr Putin and Mr Obama, don't like each other at all. I think there's a deep degree of disrespect."
The White House has ruled out a one-on-one meeting between Mr Putin and Mr Obama at the G20 summit - but the two will be forced to interact at leaders' meetings and public photo ops, and amateur body language experts are likely to have a field day.
"This is less a visit to Russia than a trip to the G20, which happens to be hosted by Russia," said a senior US official, on condition of anonymity.
"At this time there is no bilateral meeting or pull-aside expected between the presidents."
Since the start of the Syrian civil war, the United States has frequently lamented Moscow's support for President Bashar al-Assad and its decision to block any UN Security Council action to censure him or to use military action against his regime.
For his part, Mr Putin has dismissed US claims that Syrian forces used chemical weapons against civilians nearly two weeks ago. For Mr Assad to have done so would defy common sense, he said.
Clifford Gaddy, a foreign policy expert at the Brookings Institution in Washington, agreed that the Obama-Putin relationship was toxic.
"I think the way to gauge it is in terms of what are the prospects for dealing with major issues that still remain between the two countries and that may emerge," he said.
"I think it's very poor, because we simply don't have any sense of trust in the relationship, especially at the highest level, which is the most important, between the two presidents."
Still, while the atmosphere is frosty between the Kremlin and the White House, both Mr Putin and Mr Obama are pragmatic politicians acting in their own national self interests.
So it is not impossible that the sentiments currently driving US-Russian relations could change down the road, said Steven Pifer, also of the Brookings Institution.
"I think it's probably unlikely there will be a major Obama-Putin (meeting) in the next several years.
"If the Russians are prepared to engage, and they get a sense that (a) summit might do something, I think the attitude might change.
"But that rests on Putin."