Secretary of Defense Ash Carter has slammed Russia for bombing U.S.-backed opposition forces in southern Syria, calling the actions "problematic."
Speaking to reporters Friday at the Pentagon, Carter said the fighters hit by Russia's airstrikes were fighting against the Islamic State militant group. He said the Russians either intentionally attacked the rebels — which would conflict with their stated goal of defeating IS — or they fired on the rebels by accident, as a result of "poor quality" intelligence.
The secretary also suggested the Russian military did not "properly use" a communication line that was established between the two countries to prevent unsafe U.S. and Russian air operations over Syria.
A senior U.S. official said Russian warplanes had not been active for some time in the area where the U.S.-supported rebels were attacked, near al-Tanf along Syria's border with Iraq and Jordan. That raises "serious concerns" about the Russians' intentions, according to the official, who asked not to be identified.
"We will seek an explanation from Russia on why it took this action, and assurances this will not happen again," the official said Thursday.
The U.S. has criticized Russia's tactics and operational activities in Syria on a number of occasions, and has repeatedly refused to work with Russian forces in the country. American officials feel the Russians are working to prop up Syrian President Bashar al-Assad's embattled regime.
The airstrikes are likely to test already strained U.S.-Russian relations. U.S. Secretary of State John Kerry has warned both Moscow and Damascus to respect the "cessation of hostilities" agreement they signed earlier this year.
"Russia needs to understand that our patience is not infinite. In fact, it is very limited with whether or not [Bashar] al-Assad is going to be held accountable," Kerry said following a meeting Wednesday with Iranian officials.
The U.S. military has been operating a training program in Syria since early 2015 to prepare moderate Syrian rebels to fight against IS. The program experienced limited success, and the U.S. Defense Department has now switched its strategy to work with a limited number of rebels instead of entire units.