WASHINGTON (Reuters) - The United States and Russia have not reached a ceasefire deal for Syria, the U.S. State Department said on Wednesday, saying it could not confirm Moscow’s announcement that the U.S. and Russian foreign ministers would meet in Geneva on Thursday.

Russian President Vladimir Putin (L) greets U.S. Secretary of State John Kerry ahead of a meeting with U.S. President Barack Obama (not seen) on the sidelines of the G20 Summit in Hangzhou, China, September 5, 2016. Sputnik/Kremlin/Alexei Druzhinin/via REUTERS

“We’re not there yet,” State Department spokesman Mark Toner told a briefing after U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov spoke by phone for 45 minutes.

“The secretary remains committed to continuing efforts to try and resolve the outstanding issues in order to reach an arrangement on Syria ... but we won’t agree to an arrangement that does not meet our core objectives,” Toner said.

“We have not been able to reach a clear understanding on a way forward,” Toner said, adding: “I can’t say there is a big hope for success, we’re just continuing to work at it.”

Kerry and Lavrov have met twice in two weeks but failed to reach an understanding on how to proceed. After talks on the sidelines of the G20 leaders’ summit in Hangzhou, China last week, the United States accused Russia of pulling back on issues that Washington thought had been resolved.

Kerry has long expressed frustration with the lack of progress on Syria and faced criticism for trying to make a deal with Russia, an ally of Syrian President Bashar al-Assad, whom U.S. President Barack Obama has said “must go”. But Kerry sees the talks as an opportunity to end the five-year Syrian war, which has claimed an estimated 400,000 lives and driven tens of thousands of refugees into Europe.

Speaking in Hangzhou, Obama said the talks had been complicated by “gaps of trust” between Washington and Moscow, which back opposite sides in the war.

Russian and American experts have worked since July on details to halt the violence in areas where moderate opposition groups, supported by the United States and Gulf allies, and Russian-backed Syrian government forces have been battling.

Those efforts were complicated by a significant offensive in the southern part of the divided city of Aleppo where al-Qaeda- linked groups had become more intermingled with opposition fighters. Russia is insisting that opposition forces be separated from the militants.

“A lot of the sticking points focus around Aleppo, around Nusra, around delineating between where Nusra is and the opposition is, and around ... the next steps and how we get to a nationwide cessation of hostilities,” a senior State Department official told reporters.

Washington wants Syria’s air force grounded, leaving air strikes to U.S. and Russian jets that would focus on targeting Islamic State and other groups that were part of any ceasefire. An agreement hinges on Russia using its influence over Assad, and for Gulf states to convince opposition groups to take part.

Kerry’s proposal for military cooperation with Russia in Syria faces strong pushback from U.S. defense and intelligence officials who argued that Moscow cannot be trusted.

Speaking in Oxford, England, on Wednesday, U.S. Defense Secretary Ash Carter accused Moscow of aggressive behavior aimed at eroding the international order.

While Carter’s remarks followed hacking attacks on Democratic Party organizations in the lead-up to the Nov. 8 U.S. election, his comments appeared to also reflect diverging policies in Syria.

But Syria’s main opposition group, the High Negotiations Committee, said on Wednesday it would reject any deal struck by Russia and the United States that was very different from its own proposed transition group.

Riyad Hijab, the group’s general coordinator, presented a political transition plan at a meeting in London.

The U.S.-Russia proposal being discussed would make way for a political transition but does not elaborate on Assad’s future.