The UN ambassadors from the United States and Russia clashed angrily Saturday ahead of an emergency session of the UN Security Council over an airstrike by US-led coalition forces in Syria that killed dozens of Syrian government troops.

Russia demanded the council hold the meeting so that the United States could explain the airstrike, but only a short time after the meeting began in New York, the Russian ambassador walked out complaining of "American heavy-handedness."

The airstrike killed at least 83 Syrian soldiers and injured 120 in the area of Deir al-Zour in eastern Syria, the Syrian Observatory for Human Rights said. After the airstrike the Islamic State group captured an artillery battalion belonging to Syrian forces, the Observatory said.

The bombardment came as a ceasefire negotiated by the US and Russia and effect in Syria since Monday appeared increasingly fragile.

Hours after the airstrike, the United States relayed its regret to Syria through Russia "for the unintentional loss of life of Syrian forces," according to US news reports quoting an Obama administration official.

Prior to the closed-door meeting at the UN, US ambassador Samantha Power criticized Russia for calling the meeting, saying its outrage over the airstrike was a "stunt" that was "uniquely cynical and hypocritical."

Power said Russia had not called emergency meetings over Syria while the Syrian regime was attacking opposition forces or withholding aid from besieged areas.

"So why are we having this meeting tonight? It's a diversion from what is happening on the ground in Syria," Power said.

Churkin said Russia wanted the council to hear its concern, saying it was "quite significant and frankly, suspicious" that the US chose to conduct the airstrike at this time.

"It is very strange to believe that it has been an accidental thing - the timing, other aspects of the situation indicate that it may well be a provocation," Churkin said.

The Russian Foreign Ministry also said earlier that the airstrike jeopardized the Syrian ceasefire deal.

The US military said in a statement coalition forces halted the bombing operation when they learned from Russian officials that the target possibly belonged to the Syrian military.

The coalition forces struck south of Deir al-Zour, the US military's Central Command said in a statement emailed to dpa. The forces believed they were targeting an Islamic State position in an area the coalition had struck in the past, it said.

When Russian officials informed coalition officials that it was possible the targeted position belonged to the Syrian military, the coalition airstrike was halted immediately, it said.

"Syria is a complex situation with various military forces and militias in close proximity, but coalition forces would not intentionally strike a known Syrian military unit," the statement said. The US military said coalition members had informed Russian counterparts about the airstrike ahead of time.

The Syrian army said the bombardment "clearly set the scene for a terrorist attack by Daesh on the position and controlling it," using an Arabic acronym for Islamic State.

Since the ceasefire took effect, government forces and rebels groups have traded accusations of truce violations, and aid groups are still waiting for delivering humanitarian supplies to besieged areas inside Syria.

Syria's Foreign Ministry denied Saturday that government of President Bashar al-Assad was blocking aid access to the divided city of Aleppo, Syria's pre-war commercial hub, has been divided between al-Assad's forces in the west and rebels to the east since fighting erupted for the control of the city in mid-2012.

Earlier Saturday, a UN humanitarian group expressed frustration that it has so far been unable to deliver aid to Aleppo amid fears that the truce is about to collapse.

"As of Saturday morning, there has been no [delivery] progress," said David Swanson, an official at the UN's regional humanitarian affairs office for the Syria crisis.

Rami Abdel-Rahman, the Observatory head, expressed fears for the truce.

"Since Thursday, [ceasefire] violations have increased and intensified," Abdel-Rahman told dpa. "I have fears that this will eventually lead to a total collapse of the fragile ceasefire."

The truce excludes operations against Islamic State and Jabhat Fatah al-Sham, a former al-Qaeda-linked group previously known as the al-Nusra Front.

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