The Pentagon admitted on Saturday that the United States-led coalition fighting the Islamic State (ISIS) group in Syria may have struck Syrian military positions inside the country, after Russia accused it of killing at least 62 Syrian troops.
"Coalition forces believed they were striking a Daesh fighting position," Central Command, known as Centcom, said in a statement, using the Arabic acronym term for ISIS.
"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," it added.
The United States "regrets" the death of Syrian troops, a senior U.S. administration official said.
Washington has "relayed our regret through the Russian Federation for the unintentional loss of life of Syrian forces," he added.
Syria's armed forces said coalition aircraft hit a Syrian military position near the Deir Ezzor airport in the country's east, in what appears to have been the first time coalition forces have hit a Syrian regime target.
Syrian ally Russia said 62 Syrian soldiers were killed and at least 100 more wounded in strikes by "warplanes from the international anti-jihadist coalition."
The Russian military said two F-16 and two A-10 jets flew into Syrian air space from neighboring Iraq to carry out the strikes.
The Syrian Observatory for Human Rights monitoring group gave a toll of 83 soldiers killed, as it confirmed the strikes were U.S.-led coalition raids.
A U.S. military official also appeared to confirm the attack, saying "we believe about a half-dozen vehicles, including what we think was a tank, were struck as well as personnel who were out in the open "
The official could not give an estimated number of casualties.
The Pentagon said that coalition forces "would not intentionally strike a known Syrian military unit."
The strike comes less than a week into a fragile ceasefire aimed at stopping the bloodshed in Syria's five-year civil war, as Russia accused what it termed "moderate rebels" of causing the truce to fail.
The ceasefire was declared in Syria's five-year-old civil war on Monday and was extended by 48 hours on Thursday.
The UN Security Council is set to hold urgent consultations on Saturday evening over the coalition air strikes, following a request from Moscow.
"We demand Washington's full and detailed explanation," Russian foreign ministry spokeswoman Maria Zakharova said in Moscow.
The situation in Syria is deteriorating, the Russian military said earlier, adding that the United States would be responsible if it collapses.
The Pentagon said the coalition is reviewing the strike, stressing that Syria is "a complex situation with various military forces and militias in close proximity"
"The location of the strike is in an area the coalition has struck in the past," it said, "and coalition members in the Combined Air Operations Center had earlier informed Russian counterparts of the upcoming strike."
AFP contributed to this report.