The Britain-based monitor said the strike was conducted early Monday in the south of Hasakeh province by the US-led coalition, which has been battling IS jihadists in the region since 2014.

There was no immediate confirmation by the coalition of the strike, the latest in a series to have reportedly caused civilian casualties in the area in recent weeks.

The strike "resulted in the death of 11 civilians, including five children," Observatory chief Rami Abdel Rahman told AFP. "The bodies were only pulled out on Tuesday due to continuous bombardment."

He said the area targeted was near the village of Jizaa, one of the very last pockets still controlled by IS in eastern Syria's Euphrates Valley.

Kurdish-dominated forces backed by the US-led coalition are spearheading the ground offensive in the area.

The Observatory, which relies on a network of sources inside Syria, says it determines whose planes carried out raids according to type, location, flight patterns and munitions used.

The coalition assesses some reports of civilian deaths resulting from its air strikes, and releases updated figures on the casualties it admits to often weeks or months later.

On Friday, the coalition admitted to nine more civilian deaths over a period of a year, bringing the total since the start of its intervention in Iraq and Syria to 892.

Monitoring group Airwars says the number acknowledged by the coalition is well below the true toll of the bombing campaign, estimating that at least 6,259 civilians have lost their lives in both countries.