The US-led coalition has admitted it may have unintentionally struck Syrian troops while carrying out a raid against the Islamic State group on Saturday, threatening an already fragile U.S. and Russian-brokered cease-fire that has largely held despite dozens of alleged violations on both sides.
Russian army has said that airstriked killed at least 62 Syrian troops fighting IS.
U.S. Central Command said the strike in Deir al-Zour was immediately halted "when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military."
US administration has expressed "regret" for the "unintentional loss of life".
Russia immediately convened an emergency meeting of the United Nations Security Council to discuss the crisis.
If confirmed, it would mark the first known direct American strike on Syrian President Bashar Assad's forces. The United Nations Security Council held a closed emergency meeting Saturday night at Russia's request to discuss the airstrike.
The Syrian military said the deadly airstrike hit a base in the eastern city of Deir el-Zour that is surrounded by IS, allowing the extremists to advance and overrun Syrian army positions in the area. Russia's military said it was told by the Syrian army that at least 62 soldiers were killed in the air raid and more than 100 wounded.
The apparently errant strike could deal a crushing blow to the fragile cease-fire that has largely held for five days despite dozens of alleged violations on both sides. The cease-fire, which does not apply to attacks on IS, has already been the subject of disputes between Moscow and Washington, with each accusing the other of failing to fully implement it.
"Coalition forces would not intentionally strike a known Syrian military unit," the U.S. military statement said.
The Syrian military said the airstrikes enabled an IS advance on a hill overlooking the air base. It called the strike a "serious and blatant attack on Syria and its military," and "firm proof of the U.S. support of Daesh," using the Arabic acronym for IS.
A Syrian military spokesman told a briefing early Sunday that the U.S. airstrike destroyed three tanks, three infantry fighting vehicles, four mortars and an anti-aircraft gun, Russia's TASS news agency reported. The unnamed spokesman said the IS advance on the base was stopped after Russian warplanes were called in to hit IS positions.
U.S. Ambassador to the United Nations Samantha Power said the U.S. is investigating the incident and expressed regret over the loss of life as she spoke to reporters outside the Security Council chamber while the Russian ambassador was addressing the meeting.
But she accused Russia of pulling "a stunt" that is "uniquely cynical and hypocritical" in calling for the emergency meeting, while never taking similar action to condemn Assad's regime "for some of the most systematic atrocities we have seen in a generation."
Russian Ambassador Vitaly Churkin then walked out of the acrimonious meeting before Power spoke to the council. He said that in his decades as a diplomat he had "never seen such an extraordinary display of American heavy-handedness as we are witnessing today" and was "appalled" by Power's remarks.
Churkin said the U.S. airstrike put "a very big question mark" over the future of the cease-fire agreement and its timing is "frankly suspicious" because it comes just two days before the U.S. and Russia are supposed to implement an agreement on military coordination in Syria. But he held out hope that the U.S. will find a way to convince us that it's serious about finding a political solution in Syria and fighting terrorism.
In Moscow, Foreign Ministry spokeswoman Maria Zakharova was quoted by the state news agency TASS as saying that "after today's attack on the Syrian army, we come to the terrible conclusion that the White House is defending the Islamic State." Power said the Russian spokeswoman should be "embarrassed" for making such an insinuation.
The Syrian Foreign Ministry said it sent an urgent message asking the Security Council to "condemn the U.S. aggression," TASS reported, citing a statement released by Syria's SANA news. The statement claimed five U.S. aircraft took part in the airstrike.
A senior Obama administration official said the United States has "relayed our regret" for the unintentional loss of life of Syrian forces fighting the IS.
The official says the notification was sent through Russia. The official also said the U.S. will continue to pursue compliance with the cessation of hostilities as it continues military action against IS and an al-Qaida-affiliated group. The official was not authorized to discuss the notification by name and requested anonymity.
A Russian Defense Ministry official said Syria has informed them that 62 of its soldiers were killed in the airstrike. Russia has been waging a year-old air campaign on behalf of Assad's forces and closely coordinates with them.
Defense Ministry spokesman Maj. Gen. Igor Konashenkov said the airstrike near Deir el-Zour airport, held by the government, was conducted by two F-16s and two A-10s. He did not identify the planes' country affiliation, but said they were part of the international coalition. Konashenkov said the planes came from the direction of the Iraqi border, he added.
He added that if the coalition attack was launched by mistake, the reason for it was a "stubborn reluctance by the American side to coordinate its action against terrorist groups in Syria with Russia."
IS has repeatedly attacked the government-held air base, which is an isolated enclave deep in extremist-held territory. The government controls the air base and parts of Deir el-Zour city, while IS controls the entire province by the same name. An IS advance in Deir el-Zour would endanger the lives of tens of thousands of civilians living in government-held areas. The Syrian Observatory for Human Rights said the Syrian army later recaptured most of the positions it lost to IS.
The U.S.-led coalition has carried out thousands of airstrikes against IS in Syria and Iraq over the past two years, allowing allied forces on the ground to liberate several towns and cities from the extremist group. Russia also carries out attacks against IS targets, in Deir el-Zour and other parts of Syria.
"The coalition will review this strike and the circumstances surrounding it to see if any lessons can be learned," the U.S Central Command statement said.
The cease-fire took effect on Monday, and despite reports of violations, it has largely held. However, aid convoys have been unable to enter rebel-held parts of the northern city of Aleppo -- a key component of the deal.
Earlier on Saturday, Russian President Vladimir Putin questioned the U.S. commitment to cease-fire, suggesting that Washington wasn't prepared to break with "terrorist elements" battling Assad's forces.
Russia has accused Washington of failing to rein in the rebels, and on Saturday Putin asked why the United States has insisted on not releasing a written copy of the agreement.
"This comes from the problems the U.S. is facing on the Syrian track — they still cannot separate the so-called healthy part of the opposition from the half-criminal and terrorist elements," Putin said during a trip to Kyrgyzstan.
"In my opinion, this comes from the desire to keep the combat potential in fighting the legitimate government of Bashar Assad. But this is a very dangerous route."
He appeared to be referring to the Fatah al-Sham Front, an al-Qaida-linked group previously known as the Nusra Front, which is deeply embedded in rebel-held areas and fights alongside more moderate groups. Abu Mohammed al-Golani, the leader of the group, condemned the cease-fire agreement in an interview with Al-Jazeera TV aired Saturday.
Russian Foreign Minister Sergey Lavrov echoed Putin's remarks during a phone call with U.S. Secretary of State John Kerry. Lavrov noted the "refusal by an array of illegal armed groups to join the cease-fire," and Washington's obligation to "separate units of the moderate opposition from terrorist groupings," according to a Foreign Ministry statement.
Under the cease-fire agreement, the U.S. and Russia would work together to target the Fatah al-Sham Front, as well as IS, while Assad's forces refrain from striking opposition-held areas.
But Washington has warned Russia that unless aid is delivered to Aleppo, it will not move ahead with the formation of the joint coordination center.
The U.N. has accused Assad's government of obstructing aid access to the contested city. The Russian military says insurgents have held up the delivery by firing on government positions along the main route leading into besieged, rebel-held districts, in violation of the cease-fire.
The Syrian government said it has done all that is necessary to facilitate the entry of aid convoys to Aleppo, but that armed groups have failed to withdraw from the supply routes and are committing "dangerous, provocative acts."
Russia's military said Syrian rebels violated the cease-fire dozens of times over the past day, including with strikes on military and civilian targets in Aleppo. Syrian activists said government forces killed five civilians in separate attacks on Saturday.
Syria's conflict has killed more than 300,000 people and displaced half the country's population since March 2011.
(With AP inputs)