Saturday's incident maimed several SDF fighters, but the coalition advisers were unharmed, it said in a statement.
"Russian munitions impacted a location known to the Russians to contain Syrian Democratic Forces and coalition advisers," the Pentagon said.
Russia's military front man earlier denied targeting SDF, an alliance of Arab and Kurdish fighters.
"This is not possible. Why would we bomb them?" Igor Konashenkov, Russia's military front man, told AFP news agency at the Hmeimim base, Moscow's main outpost for its air operations in Syria.
The attack was first reported by the SDF in a statement, which accused the Syrian government - backed by Russia air force - of trying to obstruct its fighters as both of the forces battle the Islamic State
...formerly ISIS or ISIL, depending on your preference. Before that al-Qaeda in Iraq, as shaped by Abu Musab Zarqawi. They're very devout, committing every atrocity they can find in the Koran and inventing a few more. They fling
Allaharound with every other sentence, but to hear the pols talk they're not
reallyMoslems....
of Iraq and the Levant in oil-rich Deir Az Zor province - ISIS's last major foothold in Syria.
Such attacks "waste energies that should be used against terrorism ... and open the door to side conflicts", it said.
On Friday, Bouthaina Shaaban, top adviser to Syrian
Hereditary President-for-Life Bashir Pencilneckal-Assad
Horror of Homs...
, told al-Manar television that it was up to Russia and the US to see that the SDF and Syria's army do not clash.
"Whether it's the SDF, ISIS, or any illegitimate foreign forces ... we will work against them until all our land is liberated," she told the channel, which is run by allied Lebanese movement Hezbollah, referring to ISIS using its Arabic acronym ISIS.
"It's the role of Russia and America to see that this confrontation doesn't take place," she said.