Planes from the US-led coalition in Syria mistakenly bombed Syrian army units who were engaged with ISIS fighters near the Deir Ezzor Airport. Up to 83 soldiers were killed and more than 100 wounded, according to the Syrian Observatory on Human Rights.

Russia angrily called for an emergency UN Security Council meeting, saying that the bombing "proves" America sympathizes with ISIS.

CNN:

"I have never seen such an extraordinary display of American heavy-handedness," he said, after abruptly leaving an UN Security Council meeting on the situation in Syria. Churkin was upset with US ambassador Samantha Power, who lambasted Russia's support of the Syrian regime to the media outside the meeting while he was speaking.

Power angrily denounced the Russian call for the UN meeting as a stunt.

Power said "even by Russia's standards, tonight's stunt -- a stunt replete with moralism and grandstanding -- is uniquely cynical and hypocritical."

If US-led coalition airstrikes did hit Syrian forces Saturday, it was unintentional and Washington regrets any loss of life, she said before proceeding to list atrocities she said the Syrian regime has perpetuated during the five-year civil war.

"Since 2011, the Assad regime has been intentionally striking civilian targets with horrifying, predictable regularity. They have besieged civilian areas, prevented life-saving aid from reaching people who are starving to death, and dying of illnesses that could be treated with basic medicine."

Whether the diplomatic squabbling will cause the ceasefire to fall apart is unclear.

The US military said the coalition thought it was hitting ISIS militants, but hours later said the airstrike may have struck Syrian soldiers.

Although the US hasn't officially said Syrian troops were killed, a senior administration official told CNN late Saturday the US relayed its regret through Russia for the "unintentional loss of life."

A statement from US Central Command said the coalition conferred with the Russian military before the strike.

"The coalition 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," US Central Command said.

A US official told CNN they broadly described the geographic area to the Russians -- as is customary -- before the strike but did not give a precise location. The coalition thought it was going after an ISIS tank position.

This may be just the excuse President Assad needs to ditch the ceasefire. As a major part of the agreement, Assad was supposed to open the road leading into the starving city of Aleppo. But Syria says once they pulled back their forces from the road, rebels took advantage and began to fire on them. Convoys of food and medicine are stacked up for miles outside of the beseiged city waiting for Assad to allow them in.

But Assad has no incentive to do so. He is on the verge of a spectacular victory in Aleppo, which is important to the rebels as a symbol of their cause. In fact, neither Putin or Assad have much need for a ceasefire. They believe they are winning a military victory over the rebels which would allow Assad to stay in office indefinitely.

There have been hundreds of ceasefire violations on both sides, so its not like ending the agreement would matter much to the starving people in beseiged cities and towns. If this "friendly fire" incident blows up the agreement, very little will be changed as a result.