The United States and Russia traded fresh barbs Wednesday over allegations of arming Syria’s combatants, further straining relations at a time when the two powers are struggling to preserve unity in confronting Iran over its nuclear ambitions.

While insisting that Iran diplomacy remains on track, officials in Washington and Moscow acknowledged damage to bilateral ties a day after the Obama administration publicly accused Russia of selling attack helicopters to Syrian President Bashar al-Assad. The allegation drew a chorus of denials and denunciations from Russian leaders, some of whom accused Washington of mistaking a shipment of refurbished or repaired helicopters for new ones.

Moscow News quoted a defense expert as saying that Russia has not delivered new attack helicopters to Syria since the early 1990s. Andrei Frolov, editor of the Arms Exports journal, suggested that the U.S. allegations might be based on “a case of the repair or possible modernization of earlier delivered machines.”

Russian officials also accused the White House of hypocrisy, saying that U.S. officials had supplied arms to Syrian rebels — a charge the Obama administration denies. “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, 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 leveled 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 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 Obama 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 told reporters. “We believe that the situation is spiraling towards 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 Putin have been 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 expect to keep working closely with Moscow on Iran issues.

“The Russians have been extremely helpful on Iran,” said a senior administration official, who spoke on the condition of anonymity to discuss internal policy deliberations 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.”

Indeed, current and former administration officials argue that Russia has its own reasons for ensuring that the nuclear talks with Iran remain on track. For one thing, Russian officials are anxious to avoid further angering Sunni Muslim populations that are incensed by Moscow’s support for Assad, said Dennis Ross, who until last fall was the White House’s chief adviser on Iran.

“If they were cast as defenders of Iran, the damage to their image in the Middle East — and in their own Muslim-majority republics — would be great,” said Ross, who is counselor for the Washington Institute for Near East Policy.

Syria is Russia’s last remaining client in the Middle East, but relations between Moscow and Damascus have gone through rough patches in the past decade.

Lavrov 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,” he said.

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.

Englund reported from Moscow.