The US-led coalition on Saturday hit a Syrian army position, killing some 62 soldiers in the air strike and prompting Russia to call an emergency meeting of the UN Security Council and accusing the US of violating the recent US-Russia deal on Syria.
The US said it had mistaken the soldiers for ISIS fighters, AFP reported.
The strikes came with diplomatic tensions escalating between Moscow and Washington less than a week into a fragile ceasefire aimed at stopping the bloodshed in Syria's five-year civil war.
American officials said the US-led coalition fighting the Islamic State (IS) group may have hit Syrian military positions.
"Coalition forces believed they were striking a Daesh (IS) fighting position," a Pentagon statement said, using an Arabic acronym for the IS group.
"The coalition air strike was halted immediately when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military."
Washington said it was investigating the incident but dismissed Moscow's call for an urgent Security Council meeting as a "stunt".
"If we determine that we did indeed strike Syrian military personnel, that was not our intention. And we of course regret the loss of life," US ambassador to the UN Samantha Power told journalists as the closed-door meeting got underway Saturday night.
Power described Russia's request for the meeting as a "stunt, replete with moralism and grandstanding," saying Moscow should instead demand a meeting with the Syrian regime to press for peace.
Russian ambassador Vitaly Churkin returned by accusing the United States of violating agreements that it would not target Syrian positions.
The Russian military earlier said the situation was deteriorating, adding that the United States would be responsible if the fragile ceasefire in force since Monday collapses.
The Syrian Observatory for Human Rights monitoring group gave a toll of 83 soldiers killed, as it confirmed the strikes were US-led coalition raids.
"Warplanes from the international anti-militant coalition carried out four air strikes today against Syrian forces surrounded by IS in the Deir Ezzor air base," a Russian army statement said.
"Sixty-two Syrian soldiers were killed and a hundred others were injured in these strikes."
A Syrian military source inside the Deir Ezzor airport told AFP US-led coalition strikes hit two hilltops near the airport.
Russian military officials meanwhile lashed out at both the United States and mainstream rebels in the strongest language yet over the ceasefire struck last week in Geneva, a last-ditch effort to stop the bloodshed in Syria.
"The situation in Syria is worsening," Russian General Vladimir Savchenko said in a televised briefing earlier.
"In the past 24 hours, the number of attacks have risen sharply," with 55 attacks on government positions and civilians, killing 12 civilians, he said.
In a statement, the Russian defence ministry accused what it termed "moderate rebels" of causing the ceasefire to fail.
Under the US-Russia deal, if the truce lasts seven days and humanitarian access is granted, Moscow and Washington are to work together to target militants including IS.
"If the American side does not take the necessary measures to carry out its obligations... a breakdown of the ceasefire will be on the United States," army general Viktor Poznikhir said.
Russian President Vladimir Putin, a key ally of Syrian President Bashar al-Assad, said earlier Saturday he remained "positive" about the truce but accused rebels of "attempts to regroup".