US Secretary of State John Kerry was Tuesday to hold talks in Moscow with Russian President Vladimir Putin, seeking to narrow differences over the conflict in Syria and persuade the Kremlin to put more pressure on President Bashar al-Assad. Kerry is making his first trip to Russia since taking over as the chief US diplomat in February, on what is one of his most diplomatically delicate missions to date.

Washington has long urged Moscow -- Syrian President Bashar al-Assad's most powerful ally -- to use its sway to halt the bloodshed, accusing Russian leaders of continuing to arm the Syrian regime.

The visit also coincides with the first anniversary of Putin's return to the Kremlin for an historic third term on May 7, 2012 which heralded a new chill in relations between Moscow and Washington.

Syria is likely to top the agenda when the two men meet at around 1100 GMT with the 26-month conflict threatening to spill across the region. Later, Kerry will also meet Foreign Minister Sergei Lavrov.

"I don't know if we will get an agreement or not, but we certainly think it is worth testing and trying to find some ways forward," a senior State Department official said Monday.

With events moving on the ground in Syria, it "is a time to talk to the Russians to understand that from our side we remain committed (to a political solution), and if they are as well then we need to think about how to work operationally to make that happen," the US official said.

US officials were keen to emphasise the importance of the one-on-one meeting with Putin, which will be a rare encounter between a top American official and the Russian strongman.

Russia has long accused the West of worsening the Syria conflict by seeking to topple the Assad regime, and says Moscow is solely interested in seeing a peaceful solution to conflict that has claimed more than 70,000 lives in since March 2011.

The US and other Western states have in turn accused Russia of failing to use its influence with the regime to halt the bloodshed and keeping up military deliveries to Assad.

Kerry's visit is taking place at a moment of particular tension after Israel launched air strikes in Syria which Israeli sources said targeted Iranian weapons destined for the Lebanese militant group Hezbollah.
The Russian foreign ministry said it was "especially" concerned by the attacks, warning the violence threatened neighbouring Lebanon.

According to the Kremlin, Putin spoke to Prime Minister Benjamin Netanyahu this week by telephone about the Syria conflict but details were not disclosed.

Lavrov has also sternly warned Washington not to use claims that the Assad regime has used chemical weapons as a pretext for launching military action along the lines of the 2003 invasion of Iraq.
Further complicating the picture, UN human rights investigator Carla del Ponte said at the weekend that the rebels may have used the deadly nerve agent sarin.

But a commission of inquiry later said there was no conclusive proof and the United States said it was "highly sceptical". There are a host of other issues on the agenda of the talks, including last month's Boston Marathon bombings blamed on two brothers of Chechen descent.

More contentious dossiers include American missile defence, and rows over a ban by Moscow on American adoptions of Russian children and the Russian authorities' harassment of NGOs. "Our counterparts here have made clear they are ready to engage on Syria, but they have many issues that they want to talk about," said another State Department official.

He acknowledged it was not often American officials managed to hold direct talks with Putin, describing Tuesday's meeting as "a fantastic opportunity" for talks on the entire bilateral relationship. Kerry is due to meet civil society activists on Wednesday at the residence of Ambassador Michael McFaul at which he will hear their concerns about the lack of democratic progress.

Washington has called for a more open and democratic society, angering Putin and leading him to accuse US-backed non-governmental organisations of actively working against him.