"We are not delivering to Syria, or anywhere else, items that could be used against peaceful demonstrators," Russian Foreign Minister Sergei Lavrov told reporters in Tehran, Iran, where he was on an official visit. "In this we differ from the United States."
The Obama administration stood by the helicopter accusation, which was raised by Secretary of State Hillary Rodham Clinton in response to a question at a foreign-policy forum in Washington. Clinton cited evidence of new sales of Russian-built attack helicopters to Syria and suggested that Russian officials were concealing their support for Assad's brutal repression of the country's opposition movement.
Syria is widely reported to be using Russian-built attack helicopters in assaults against civilian protesters, but Clinton's comments were the first by an administration official alleging that new Russian helicopters were heading toward Syria.
Clinton repeated the assertion Wednesday, brushing aside suggestions from Russian diplomats that U.S. spies had spotted evidence of shipments of helicopter parts, not whole helicopters.
"We know - because they confirm - that they continue to deliver," Clinton said at a State Department news conference. "We believe that the situation is spiraling toward civil war."
The diplomatic tiff comes at a sensitive time for U.S.-Russian relations, as the two countries hone their strategy ahead of nuclear talks with Iran scheduled to begin Monday in the Russian capital. Despite differences over economic sanctions against Iran, President Obama and Russian President Vladimir V. Putin have remained united in demanding strict limits on Iran's nuclear activities.
The two powers, joined by Britain, China, France, and Germany, are expected to press Iran next week to agree to freeze production of a type of enriched uranium that can be easily converted to fuel for nuclear weapons.
Administration officials said they expected to continue working closely with Moscow on Iran issues, even as the two countries sparred publicly over their approach to Syria.
"The Russians have been extremely helpful on Iran," said a senior administration official who spoke on condition of anonymity to discuss internal policy discussions on Iran. "We've been able to disagree with Russia in the past while continuing to work closely in areas where we have common interests."
Lavrov, in Tehran, again defended Russian military sales to Syria, saying Moscow was merely "completing the implementation of contracts that were signed and paid for a long time ago," he said. "All these contracts concern exclusively antiaircraft defense."
In contrast, he said the United States "regularly delivers riot-control equipment to the region, including a recent delivery to a Persian Gulf country," an apparent reference to Bahrain, which used U.S.-made riot gear in repressing a Shiite-led uprising last spring.