Doug Stanglin, and Jim Michaels

USA TODAY

The U.S. military said it halted a series of airstrikes Saturday in eastern Syria after Russia warned the United States that it mistakenly struck and killed Syrian military forces.

The Russian Defense Ministry said 62 Syrians were killed in the strike, and the Syrian government said the coalition airstrike allowed the Islamic State to advance around Deir el-Zour province where the attack took place.

Samantha Power, the U.S. ambassador to the United Nations, said U.S. forces halted the attack when informed by Russia it was possible they were striking Syrian regime personnel and vehicles. She said U.S. officials were investigating the incident.

Speaking at U.N. headquarters in New York Saturday evening, Power said, “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.”

The accusations come only days after the United States and Russia reached a fragile ceasefire agreement in Syria that was designed to lead to broader military cooperation between the two countries in Syria.

It is not clear yet what impact the incident would have on the ceasefire, which was earlier hailed by U.S. Secretary of State John Kerry as a potential turning point in the five-year old civil war.

The ceasefire does not apply to attacks on the Islamic State or al-Qaeda affiliates in Syria. The ceasefire applies to Syrian government forces and moderate opposition rebels fighting them.

Coalition forces "would not intentionally strike a known Syrian military unit," according to a statement from U.S. Central Command, which oversees operations in the Middle East. "The coalition will review this strike and the circumstances surrounding it to see if any lessons can be learned."

The U.S. military said it believed it was attacking Islamic State forces with the airstrikes.

Under the agreement, if the ceasefire holds for a week and humanitarian supplies reach needy civilians, then the U.S. and Russia's armed forces would begin exploring broader military cooperation. The ceasefire went into effect Monday.

The Syrian General Command, in its statement, called the purported attack on the Syrian Arab Army a "serious and blatant aggression" against Syria and its army, the Syrian Arab News Agency reported.

Central Command said “the airstrike 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.”

Key terrorist operative killed by Pentagon in Syria

U.S. officials said their forces had hit Islamic State, or ISIL, forces in the same area in the past and had been tracking them for a significant amount of time before the latest strike.

Pentagon Press Secretary Peter Cook said in a statement that the coalition's air operations center had notified the Russians that coalition aircraft would be operating in the area at the time and the Russians had not voiced concerns.

The U.S. military regularly talks with the Russians in order to avoid mishaps in the air, but the two countries don't cooperate on targets or military operations.

The Syrian General command charged that the U.S.-led strike in the al-Tharda Mountains around the Deir El-Zour airport in eastern Syria "paved the way for Islamic State terrorists to attack the position and take control of it."

It said the attack showed that the U.S. and its allies were not in fact interested in fighting terrorism.

In Moscow, Russia's Defense Ministry spokesman, Major-General Igor Konashenkov, said the aircraft that carried out the bombings — two F-16 jet fighters and two A-10 support aircraft — had entered Syrian airspace from Iraqi territory. The U.S. military did not comment on the Russian claims.

Even if the bombardment of the Syrian government troops was a mistake, the spokesman said, it is still a consequence of Washington’s unwillingness to coordinate its anti-terror efforts with Moscow.

The Fatah al-Sham Front, linked to al-Qaeda and previously known as the Nusra Front, is deeply embedded in rebel-held areas and fights alongside more moderate groups.

Tensions with Russia

The incident could make it difficult for Washington to press Moscow on the ceasefire. Russian President Vladimir Putin on Saturday questioned the U.S. commitment to the shaky, week-old ceasefire, suggesting that Washington wasn’t prepared to break with “terrorist elements” battling Assad’s forces.

Russia called a closed-door meeting about the airstrikes at the United Nations Security Council Saturday evening.

Russia's ambassador to the United Nations Vitaly Churkin told the assembled press "it is quite significant and frankly, suspicious, that the United States chose to conduct this particular airstrike at this time," he said.

Power later left the meeting and in remarks to reporters excoriated Russia for its reaction to the incident.

“Even by Russian standards, tonight’s stunt, a stunt replete with moralizing and grand-standing, is uniquely cynical and hypocritical,” she said.

For five years the Syrian regime has intentionally struck civilian targets, prevented life-saving food and aid from reaching people, tortured tens of thousands and used chemical weapons against its own people. In addition, it has "patented the practice of double tap strikes, hitting a civilian target, waiting for heroically brave individuals to run into collapsing buildings, burning buildings and then striking again so they can hit the rescue personnel,” she said.

“And yet in the face of none of these atrocities has Russian expressed outrages, nor has it demanded investigations. Nor has it ever called for a Saturday night emergency consultation in the Security Council,” she said.

After her remarks, Churkin walked out of the meeting.

Both sides have alleged dozens of violations of the ceasefire agreement, and aid convoys have been unable to enter rebel-held parts of the northern city of Aleppo — a key opposition demand.

Washington has warned Russia that unless aid is delivered to Aleppo, it will not move ahead with the formation of the joint implementation center, the first step toward military cooperation between Russia and the United States in Syria.

The U.N. has accused Assad’s government of obstructing aid access to Allepo. 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,” the Associated Press reports.

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.

The Interfax news agency quoted Col. Sergei Kopytsin as saying Saturday that mortar fire and homemade rockets struck Aleppo 26 times. Russian news agencies cited another official, Lt. Gen. Vladimir Savchenko, as saying there had been 55 violations throughout the country. Syria’s state news agency SANA said insurgents have violated the cease-fire 12 times in the last 12 hours. No casualties were reported.