A team of US Special Operations Forces were advising US-backed Syrian Democratic Forces at the location the Russians struck in Deir Ezzor in eastern Syria, one US official said.
US personnel were "at most a couple of miles" away from the where the bomb landed, the official added.
The US troops provided medical care to members of the Syrian Democratic Forces that were wounded in the airstrike, an image of which was tweeted out
by the coalition.
The US is still looking at the possibility the strike so close to US troops, and the SDF fighters it supports, may have been an error by the Russian pilot or a bomb that went astray rather than a deliberate attack on US troops by the Russians, the US official said.
Regardless of the circumstances of the strike, several US officials tell CNN it underscores the risk faced by having Russian, Syrian regime, US and US-backed fighters operating in extremely close proximity in eastern Syria.
The US-led coalition fighting ISIS, Operation Inherent Resolve, issued a statement Saturday saying "Russian munitions impacted a location known to the Russians to contain Syrian Democratic Forces and coalition advisers."
A coalition official told CNN that the coalition denied a Russian military request to strike an area in the province of Deir Ezzor, as there were coalition advisers and US-backed Syrian forces there. But the Russians decided to carry out the strike anyway.
Russian Defense Ministry officials denied the Pentagon's charge Sunday, saying the airstrikes were directed at ISIS targets, according to state-run news agency TASS.
"The Russian Aerospace Force inflict pinpoint strikes only on targets, confirmed through multiple channels, in the districts controlled by IS," ministry spokesman Igor Konashenkov said.
He added that Russian command "notified in advance the US counterparts about borders of the military operation" in Deir Ezzor.
The incident sparked high level discussions Sunday between US Secretary of State Rex Tillerson and Russian Foreign Minister Sergey Lavrov, as well as a phone call between the Chairman of the Joint Chiefs of Staff, Gen. Joseph Dunford and his Russian counterpart Gen. Valery Gerasimov, the Russian Chief of General Staff.
Coalition officials have said that the recent increased proximity between Russian-backed regime troops and coalition-backed Syrian Democratic Forces has necessitated increased "de-confliction" between the Russian and coalition militaries.
"The convergence of military forces in this area calls for increased awareness and de-confliction," coalition spokesman US Army Col. Ryan Dillon told reporters last week, referring to the regime and SDF offensives in Deir Ezzor province.
While regime forces had primarily operated on the western side of the Euphrates River and SDF forces on the eastern side, the Russian Ministry of Defense issued a statement Monday saying Syrian regime troops, backed by Russian airpower, had crossed over to the river's eastern bank near Deir Ezzor city.
But with Russian and coalition military officers already using the communications hotline that allows the two sides to de-conflict their respective forces on a daily basis, it is difficult to imagine how such efforts can be increased.
"Coalition officials are available and the de-confliction line with Russia is open 24 hours per day," Coalition commander Lt. Gen. Paul E. Funk said in a statement following Saturday's strike.
"We put our full efforts into preventing unnecessary escalation among forces that share ISIS as our common enemy," Funk added.
A US official told CNN that the intent was to shift the de-confliction conversations and focus them to be more tactical, sharing where ground forces were located to prevent any accidental targeting.
And while coalition officials had recently trumpeted de-confliction efforts with Russia, Saturday's incident appeared to cast doubt on their effectiveness.