Russian cruise missiles aimed at Syria reportedly landed in Iran on Thursday.
The mistaken bombing, launched from a Russian warship in the Caspian Sea, was first reported by CNN, which received the information from two unnamed U.S. officials.
From the Caspian Sea, the clearest path for the missiles to reach Syria would be by crossing over both Iran and Iraq. The officials told CNN that Russia is testing out its Kaliber missiles in combat for the first time.
The report is consistent with Iranian media accounts of an explosion in northwestern Iran. The Islamic Republic Press Agency, an official Iranian government press outlet, reported on Thursday that the governor of Takab said an "unidentified flying object" near the town crashed and then exploded. The article contained a photo of a drone, however, and did not provide details about the origins of the crash and explosion.
Iranian journalists have subsequently tweeted these government reports as evidence that that is where the bombing occurred.
It is not clear if there were any Iranian casualties from the bombing.
The purported bombing error is likely to be an embarrassment for Russian President Vladimir Putin, who began bombing Syrian rebel targets on Sept. 30. Russia hopes to shore up ally Syrian President Bashar Assad. It says it shares the United States' goal of destroying the Islamic State, but it has targeted other rebel groups as well, eliciting criticism from the U.S.
At a NATO press conference in Brussels on Thursday, Defense Secretary Ashton Carter condemned Russian bombing of nonextremist rebel groups in Syria, calling them a "fundamental strategic mistake" that "will inflame and prolong the Syrian civil war."
"We have not and will not agree to cooperate with Russia so long as they continue to pursue this misguided strategy," Carter said. U.S. and Russian officials have been meeting to try to avoid accidents between the two countries' warplanes, but Carter called the Russian military's conduct "increasingly unprofessional" and accused them of not adequately communicating with the U.S.
"They violated Turkish airspace, which as all of us here made clear earlier this week, and strongly affirmed today here in Brussels, is NATO airspace. They've shot cruise missiles from a ship in the Caspian Sea without warning," he said. "They've come within just a few miles of one of our unmanned aerial vehicles."
Carter argued that Russia's focus on propping up Assad was shortsighted, even from its own national interests, and predicted that Russians would begin to suffer casualties in Syria in the "coming days."
Iran has also backed the Syrian government in its four-year campaign to crush a popular rebellion.
A Pentagon spokesperson declined to comment on the incident. The Iranian and Russian governments did not immediately respond to request for comment.