The Pentagon has engaged with the Russian military after it struck Pentagon-supported rebels with airstrikes in southern Syria on Thursday, a senior Pentagon official said Friday.
"We have engaged with the Russians per our [memorandum of understanding] guidelines, and those conversations continue," the official said.
Reuters reported Friday afternoon that even after the Pentagon had asked Moscow to stop after the first strike, the Russians launched a second strike against the rebels.
A U.S. official told Reuters that a small number of Syrian fighters were killed in Thursday's airstrikes.
A senior defense official said Thursday that Russia's actions "raise serious concern about Russian intentions."
"We will seek an explanation from Russia on why it took this action and assurances this will not happen again," the official said.
Earlier in the day, Pentagon press secretary Peter Cook said if the reports of airstrikes were true, it would be a "mistake."
"If this indeed was a Russian airstrike, that they're targeting forces that are trying to get rid of ISIL, and we think that would be a mistake," he said, using another acronym for the Islamic State in Iraq and Syria.
Russia launched an air campaign in Syria last September, which it said was aimed at targeting ISIS. The majority of its airstrikes, however, have been aimed at Western-backed rebels in order to shore up Syrian President Bashar al Assad.
The senior defense official also pointed out that the strikes were not in support of Syrian or Russian troops.
"Russian aircraft have not been active in this area of Southern Syria for some time, and there were no Syrian regime or Russian ground forces in the vicinity," the official said.
The Obama administration has sought to avoid a military conflict with Syrian or Russian forces, leery of getting bogged down in a new war. Instead, it has tried to stop Russian airstrikes by encouraging ceasefires and a political solution to the Syrian civil war. However, neither have gained traction, and Assad has struck an increasingly defiant tone in refusing to step down.
The Pentagon did not identify the rebels but said they included individuals who have received U.S. support.
The Kremlin on Friday said it was hard to distinguish between moderate and Islamist extremist rebels on the ground because they were fighting so close to one another, according to Reuters.
Defense Secretary Ash Carter on Friday during a press conference called the Russian strikes "problematic."
"The Russians initially said they were coming in to fight ISIL, and that's not what they did," he said.
He also said the channels established in the memorandum of understanding between the U.S. and Russian militaries to avert accidents between the two militaries "wasn't professionally used."
"So we're trying [to] clarify the facts and use that channel with the Russians to understand what went on there," he said.