By Jill Dougherty, CNN Foreign Affairs Correspondent
Secretary of State John Kerry departed for Russia on Monday, as the conflict in Syria heads into a new and potentially more dangerous phase, and the Obama administration tries to pin down who used chemical weapons.
Carla Del Ponte, a member of the United Nations Independent International Commission of Inquiry on Syria, said Monday there are "strong suspicions... if not yet, let's say, indisputable proof" that sarin gas was used in Syria by opposition forces, rather than by the regime of Bashar al-Assad.
The U.N. commission later issued a statement saying it "has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."
Briefing reporters just before Kerry departed Washington for Moscow, a senior State Department official said, "Our understanding has been that the armed opposition does not have such weapons. And so we'll have to re-check our facts. But our initial take on that is that they do not have such things in their arsenal."
But Del Ponte's comments could complicate Kerry's discussions on Syria with the Russian leadership. Moscow's support for al-Assad is waning, but Russian officials have said they still have grave concerns that if al-Assad is toppled, it would mean victory for Islamic extremists.
Kerry's mission in Moscow is to convince Russia to pressure al-Assad and his supporters to accept a political settlement. But the prospects of Moscow's cutting al-Assad loose seem dim.
Syria is just one challenge for Kerry in Russia. As another senior State Department official put it, "This is one of the most complex and comprehensive bilateral relationships the U.S. has," and encompasses difficult issues including Iran, North Korea, Afghanistan, anti-terrorism cooperation and trade.
That official said, in spite of the tension over Syria, the Russians "want the trip to go well" and there is more "intensified dialogue" between the two countries than there was a year ago when both Barack Obama and Vladimir Putin were distracted by presidential elections.
In April Obama sent Putin a letter, delivered by his national security adviser, Tom Donilon, which laid out expectations for relations between Moscow and Washington over the coming months, and even years.
Planning is one thing, but an unexpected event - the terrorist bombing in Boston - has brought the countries together in what that State Department official called a "new era" in counterterrorism.
Russia's Federal Security Service, or FSB, tipped off the FBI to its suspicions about one of the Boston bombing suspects, Tamerlan Tsarnaev, back in 2011. The FBI investigated Tsarnaev, interviewing him and family members, but said it found nothing "derogatory" and closed the case.
The FBI has said it asked the Russians for more information about Tsarnaev but its inquiries went unanswered. Tsarnaev visited Russia - the troubled Caucus republics of Dagestan and Chechnya - for six months in 2012, a period that's come under much scrutiny amid questions about Tsarnaev's radicalization. Some U.S. lawmakers have criticized Russia for not providing more information to U.S. authorities.
In Moscow, Secretary Kerry will meet with President Putin at the Kremlin Tuesday and then drive to the suburbs for a meeting and dinner with Russian Foreign Minister Sergey Lavrov.
The meeting with Putin is key. The Russian president, while insisting he wants more cooperation on fighting terror, has not minced words in accusing the United States of causing a "cooling in relations."
During a recent call-in show with Russian citizens, he complained bitterly about what he perceives as a double standard: "I have always felt outraged when our Western partners, as well as your colleagues from the Western media, referred to our terrorists who committed brutal, bloody, appalling crimes on the territory of our country, as 'insurgents.'"
Later in the show he slammed the Magnitsky Act, recently pass by the U.S. Congress, which prohibits entry into the U.S. by Russian officials thought to be involved in the death of a Russian lawyer.
"It was an excellent opportunity to leave the Cold War behind and move on," Putin said. "But no, they had to think up another anti-Russian law, the Magnitsky Act. The investigation of those events has not even been completed. Why was this done? Just to show off who is the toughest here. What for? It is an imperialist approach to foreign policy."
In Moscow, John Kerry has his work cut out for him.