Air strikes, thought to be by planes from a U.S.-led military coalition, killed at least 30 people in the eastern Syrian province of Deir al-Zor on Monday, including women and children, residents and activists said.
Coalition spokesman U.S. Air Force Colonel John Dorrian confirmed that the U.S.-led coalition had conducted strikes in the vicinity of the town of al-Bukamal, but said he could not "confirm the veracity of allegations of civilian casualties".
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He told Reuters the coalition tried to avoid civilian deaths in its bombing campaign against Islamic State militants in Syria and neighbouring Iraq.
The border town has been a haven in recent years for thousands of displaced Syrians from Aleppo and from other areas, including Iraq, where its residents have strong tribal ties across the border.
An activist in touch with relatives in al-Bukamal said at least three homes had been flattened in the residential Hay al Masriya district of the town and at least 30 people, mostly women and children from six families, had been killed.
A second former resident of the town gave a similar figure and said it was likely to rise, with several critical cases among the scores of people injured.
Amaq news agency, which is affiliated to the militants, released a video that it said showed extensive damage to a whole string of houses inside the city with rescuers treating children.
There were other casualties in raids on several villages near al-Bukamal.
Earlier the Britain-based Syrian Observatory for Human Rights reported that jets "believed to belong to the international coalition" had struck al-Bukamal on Monday, killing three militants and 13 civilians including children.
The monitor said strikes also killed seven civilians in the town of al-Husainiyah further north along the Euphrates river.
Islamic State militants control most of Deir al-Zor province, which links territory they hold in Syria and Iraq, and parts of the provincial capital, which has the same name.
The Syrian government still controls some parts of Deir al-Zor city, including a nearby military air base, where Islamic State has besieged about 200,000 people lacking food and medicine for around tw years.
Syrian government forces and their allies, backed by Russian air power, have been fighting back against Islamic State assaults in the area.
The U.S.-led coalition has in recent months stepped up targeted raids on the province to rupture the militants' supply lines across Iraq and Syria.