A US airstrike, targeting what were thought to be Islamic State militants in eastern Syria, instead hit Syrian military positions Saturday, killing what Russia and the Syrian government said were 62 soldiers and injuring about 100.

The U.S. Central Command acknowledged the strike, saying it was "halted immediately" when U.S. forces were informed by Russia "that it was possible the personnel and vehicles targeted were part of the Syrian military." The command statement said Russia had been notified beforehand as a "professional courtesy" that U.S. aircraft would be in the area, but U.S. officials said no reply had been received.

A U.S. Defense official said the strike "appears to be an intelligence failure."

It marked the first time the United States has engaged the Syrian military since it began targeting the Islamic State in Syria and Iraq two years ago. The strike comes at a particularly sensitive time in U.S. and Russian efforts to forge a cease-fire in Syria's civil war, as each has accused the other of failing to comply with an agreement they struck a week ago for a ceasefire.

Although the Central Command did not mention casualties, a senior official in President Barack Obama's administration said that "the United States has relayed our regret through the Russian Federation for the unintentional loss of life of Syrian forces fighting ISIL." ISIL and ISIS are alternative names for the Islamic State.

Russia called for an emergency meeting of the United Nations Security Council, to take place later Saturday evening.

Deir al-Zour province, where the strike took place, is far from the populated western region where the separate civil war that is the subject of the cease-fire is focused. But administration officials acknowledged that the incident was "not helpful" to efforts to stop that fighting, deliver humanitarian aid to besieged Syrians and ultimately coordinate counterterrorism efforts with Russia under the new agreement.

The senior administration official said that the United States would continue to "forcefully" pursue compliance with the deal, even as "we continue military action against ISIL and Al Qaida" in Syria.

The Islamic State controls much of the eastern province, although there are some widely scattered Syrian military installations in the scantily-populated area and near the city of Deir al-Zour. Russian aircraft have occasionally struck in the area, and the United Nations has organized air drops of relief supplies to the population of the Islamic State-surrounded city.

Militant forces in the region have been repeatedly hit by U.S. airstrikes, and the Central Command statement said that U.S. surveillance had been "tracking" an Islamic State fighting position "for a significant amount of time before the strike." The Defense official, among several officials who spoke on condition of anonymity to discuss the developing situation, said tracking had gone on for "some days," and that the militants appeared to have a number of vehicles and a tank.

Under the Tampa-based Central Command, coalition airstrikes against the Islamic State in Iraq and Syria are coordinated through planning cells located in the Middle East. Target tracking is done through a variety of means, including aerial reconaissance by surveilance drones, and communications intercepts.

The Defense official said that before the strike was called off, U.S. forces had destroyed roughly six vehicles and the "personnel associated with them." The official would not comment on the alleged casualty numbers, and denied Syrian news reports that U.S. aircraft dropped incendiary bombs on the Syrian forces. The U.S. aircraft, the official said, used only guided bombs.

"If we did get this wrong, which it looks like we did, it's not something we intended to do," the official said, adding that the Central Command plans to investigate how targeting personnel could have confused a Syrian military unit with Islamic State fighters.

In a statement reported by the state-run news agency, the Syrian military said its troops had been surrounded by Islamic State fighters, and the U.S. strike "paved the way for ISIS terrorists to attack" a nearby hilltop. In details the Pentagon did not confirm, it said the strike was carried out by four U.S. jets--two F-16 fighters and two A-10 ground-attack aircraft--flying from the Iraqi border.

The statement said the airstrikes were "conclusive evidence that the United States and its allies support ISIS and other terrorist organizations." The Assad government considers all forces opposed to it to be terrorists.

Russia last year began its own airstrikes in Syria, primarily in support of Assad's forces fighting the civil war, but also occasionally targeting the Islamic State. Last fall, the United States and Russia signed a "deconfliction" agreement to share enough information about where their planes were flying to keep them away from each other.

But the risk of potential collisions has, for the most part, been in western Syria. The Central Command statement said U.S. forces were not required, under the deconfliction agreement, to inform Russia of the planned Saturday strike. But, it said, "it is not uncommon for the Coalition Air Operations Center to confer with Russian officials as a professional courtesy and to deconflict" their respective aircraft.

"Syria is a complex situation with various military forces and militias in close proximity," the statement said, "but coalition forces would not intentionally strike a known Syrian military unit. The coalition will review this strike and the circumstances surrounding it to see if any lessons can be learned."

Although fighting has diminished in the western part of the country where the civil war is taking place, no aid deliveries have yet begun because of what the United Nations has said is the government's failure to guarantee the safety of the truck convoys.

Over the past week, the United States and Russia have exchanged increasingly hostile charges of non-compliance with the cease-fire agreement reached in Geneva last weekend by Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov. Administration officials said Friday that it was increasingly clear that Moscow did not have the influence it thought it had over Syrian President Bashar Assad.

Moscow has countercharged that the United States has failed to separate opposition forces it supports from forces of anti-Assad terrorist groups, particularly those of the former al-Qaida militia, which recently changed its name from Jabhat al-Nusra to Jabhat Fatah al-Sham, or the Front for the Conquest of Syria.

Under the agreement, a U.S.-Russia Joint Implementation Cell to coordinate strikes against the Islamic State and al-Qaida will not be set in motion until there are seven consecutive days of reduced fighting and aid deliveries.

Russia has been anxious for the coordination to begin, and on Saturday, quickly saw a propaganda advantage in the Deir al-Zour strike. "If this airstrike was caused by target location error, then this is a direct consequence of the U.S. side's stubborn reluctance to coordinate with Russia its actions against the terrorist groups operating in Syria," Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said.

Kerry spoke with Lavrov by telephone Saturday morning, before reports of the airstrike. Lavrov repeated its insistence that the United States release the documents detailing the agreement, saying that the secrecy has promoted "liberal and sometimes distorted interpretations" of the deal.

"Our view is that it's not prudent at this time to make this public," another senior administration official said. "There's a concern about some of the specificity of information, and what that may convey to people who are not party" to the cease-fire.