After authorizing for the first time supplying U.S. weapons to Syrian rebels, President Barack Obama will face difficult talks next week with Russian President Vladimir Putin, an important ally of Syria's government, on ways to end the civil war there.
Obama on Monday is set to hold his first private, face-to-face meeting with Putin in a year at the G8 conference in Northern Ireland, the White House said on Friday.
Obama will discuss U.S. evidence that Syrian President Bashar al-Assad's forces have used chemical weapons - what the U.S. president has called a "red line" in the two-year-old civil war, the White House said.
Russia's Foreign Minister Sergei Lavrov has told Secretary of State John Kerry that U.S. accusations of chemical weapons use by Assad's forces "are not backed up by verified facts," the Russian foreign ministry said on Friday. Lavrov also said more U.S. military support for Syrian rebels could escalate violence in the region, the foreign ministry added.
Obama has authorized sending U.S. weapons to Syrian rebels, a U.S. official said on Thursday.
The United States has provided details to Russia on several chemical weapons attacks, said Ben Rhodes, Obama's national security spokesman.
"We've got physical samples that demonstrate that point," he told reporters at a White House briefing.
Obama wants to talk to other G8 allies including Britain and France about the situation on the ground in Syria and the type of support the United States plans to provide the opposition forces, Rhodes said.
"He'll be discussing with those leaders what the best way forward is. He'll hear from them what their plans are," he said.
Rhodes said Obama will focus on finding common ground with Putin on talks toward a political settlement in Syria. "There are no illusions that that's going to be easy," Rhodes said.
Putin has backed Assad, while Obama has maintained Assad cannot be part of any political settlement to the conflict.
"What Russia has articulated to us, and publicly, is that they don't want to see a downward spiral, they don't want to see a chaotic and unstable situation in the region, they don't want to see extremist elements gaining a foothold in Syria," Rhodes added.
Russia has been Assad's most powerful protector during the civil war, opposing sanctions and, with China, vetoing three U.N. Security Council resolutions aimed at putting pressure on Assad's government or pushing him from power.