Russia has increased its weapons shipments to the Syrian government in the past year, complicating efforts to start peace talks, the U.S. ambassador to Syria said Thursday.

Ambassador Robert Ford called the Russian military aid “substantial” and in some cases “militarily extremely significant.” A second U.S. official, Assistant Secretary of State Thomas Countryman, said the Russian aid is now more important than the weapons provided to President Bashar Assad’s forces by Iran.

While he didn’t provide details, Ford made clear that Russia’s action are a cause for tension between Secretary of State John Kerry and his Russian counterpart, Sergei Lavrov. They speak several times a week in efforts to prepare for peace talks in Geneva as soon as next month, Ford told the Senate Foreign Relations Committee.

“We have had, including at the level of the secretary, a lot of discussion with the Russians” about the arms shipments, Ford said. “The Russians would help everyone get to the negotiating table faster if they would stop these deliveries.”

Sen. John McCain, R-Ariz., said the United States in an “Orwellian situation” in which Russia cooperates on eliminating Syria’s chemical weapons while increasing the quantity and quality of the conventional weapons in the hands of Assad’s forces.

Tennessee Sen. Bob Corker, the committee’s senior Republican, said the United States appears “feckless” with its slow and limited military supplies to the Syrian rebels. He said the U.S. has covertly trained about 1,000 Syrian rebels, and is continuing at a rate of 50 to 100 a month.

Countryman, assistant secretary for international security and nonproliferation, told the committee that Russia’s “unswerving support” for the Syrian government “costs the Russians in credibility with the rest of the Arab world and with the entire region.”

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