WASHINGTON (AP) — The Obama administration came back empty-handed after a diplomatic delegation to Moscow last month presented intelligence suggesting that Syrian President Bashar Assad’s government used chemical weapons on its own people, officials said Tuesday, raising further questions about how seriously the United States can cooperate with Russia to end the Arab country’s civil war.

The U.S. officials said that while some of the information presented wasn’t new, it reflected the ongoing effort by the United States to persuade Russia to drop its support for the Assad regime two years into a conflict that has claimed more than 70,000 lives. One official described it as the U.S. government’s “best evidence” of chemical weapons use by regime forces, even if President Barack Obama and others in the administration say they still lack incontrovertible proof of chemical weapons attacks, which would represent a crossing of Obama’s “red line” for potential military action.

On Tuesday, French Foreign Minister Laurent Fabius said tests carried out by a French laboratory confirmed that sarin gas has been used multiple times and at least once by Syrian government forces and their accomplices. Earlier Tuesday, a U.N. panel said there were “reasonable ground” to believe limited quantities of toxic chemicals were used as weapons in at least four attacks in Syria’s civil war, but that more evidence was needed to determine the precise chemical agents involved and who used them.

The U.S. officials, who weren’t authorized to speak publicly on the matter and demanded anonymity, said the intelligence failed to convince Russian officials and prompted no change in the Kremlin’s support for Assad — a disappointment for the U.S., considering the promises of closer cooperation after several recent meetings and conversations between Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov.

Sitting across from Lavrov in Paris last week, Kerry told reporters that both men expressed concerns “about any potential use of chemical weapons and the need to really get the evidence and ascertain what has happened in that regard. Both Russia and the United States, if it were being used, object to that very, very strongly.”

Kerry and Lavrov have been engaged in an intensive effort to get Syria’s government and opposition into peace talks, even as Moscow has continued to provide Assad with military aid.

State Department spokeswoman Jen Psaki said the two top diplomats discussed plans to share information on alleged chemical weapons use when they met one-on-one in the Russian capital a month ago but didn’t address specific evidence in their conversations. Senior U.S. and Russian officials are “continuing to coordinate plans to share information and hope to do that in the short term,” she said.

Russia’s unwillingness to accept what appears to a growing international consensus on chemical weapons use in Syria comes on the heels of other decisions that have frustrated Washington. U.S. officials have chastised Moscow for delivering to Assad’s government anti-ship missiles and potentially a state-of-the-art air defense system that would make a U.S.-enforced no-fly zone or any other military intervention significantly more difficult. They’ve also expressed concerns about what the upgraded equipment would mean for the security of Israel, across Syria’s southwestern border.

Nevertheless, Obama administration officials have tried to remain hopeful about the peace push with Russia. They’re hoping to launch direct negotiations between representatives of Assad’s government and the Syrian opposition in Geneva. The conference, once foreseen for May and then delayed until June, is now not expected to happen until July at the earliest. In the meantime, Syrian government forces have been making significant advances against the rebellion.

Copyright 2013 The Associated Press.