The US military says coalition forces allied in a fight against Islamic State stopped a bombing operation in eastern Syria when they learned from Russian officials that the target possibly belonged to the Syrian military.
The Syrian Observatory for Human Rights said that the airstrikes on Saturday (local time) killed at least 83 soldiers of the Bashar al-Assad regime and injured 120 in the area of Deir al-Zour in eastern Syria.
The monitoring group confirmed that after the airstrike Islamic State attacked and captured an artillery battalion.
The Russian Defence Ministry said earlier that coalition warplanes had killed 62 Syrian servicemen and injured 100 others, TASS reported.
The bombardment comes as a truce brokered by the US and Russia, which went into effect in Syria on Monday (local time), appeared increasingly fragile.
The coalition forces struck south of Deir al-Zour, the US military's Central Command said in a statement emailed to media. The forces had been tracking the position for a significant amount of time before the strike, and the location was 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.
The Syrian army said the bombardment "clearly set the scene for a terrorist attack by Daesh (Islamic State) on the position and controlling it".
Government forces and rebels groups have traded accusations of truce violations, and aid groups are still waiting to deliver humanitarian supplies to besieged areas inside Syria.
Syria's Foreign Ministry denied on Saturday that Assad regime was blocking aid access to the divided city of Aleppo.
Aleppo, Syria's pre-war commercial hub, has been divided between Assad regime forces in the west and rebels to the east since fighting erupted for the control of the city in mid-2012.
Earlier on Saturday, a UN humanitarian group expressed frustration that it has so far been unable to deliver aid to Aleppo amid fears that the US-Russian 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.
Dozens of trucks with UN relief supplies remain stuck on the Turkish border.
Rami Abdel-Rahman, the Observatory head, expressed fears for the truce, which is the second this year to be brokered by Washington and Moscow. The first took effect in February but failed to hold.
"Since Thursday, [ceasefire] violations have increased and intensified," says Abdel-Rahman. "I have fears that this will eventually lead to a total collapse of the fragile ceasefire."
The truce excludes Islamic State and Jabhat Fatah al-Sham, a former al-Qaeda-linked group previously known as al-Nusra Front.