An air strike by the US-led coalition fighting ISIS targeted a mosque in Syria last week because it was determined to be an insurgent "command-and-control centre", the US said on Sunday.
It denied that it had targeted civilians in the deadly raid in eastern Syria.
The coalition said that while the law of war protects mosques, the use of the building as a headquarters by ISIS caused it to lose that protected status. It said a dozen fighters were killed.
US Army General Joseph Votel, the top American commander for the Middle East, said that he is satisfied that commanders went through the proper procedures to determine that the mosque was being used by ISIS and that it was a legitimate target.
"The determination that was made by the leadership on the ground was that this mosque was not being used as a mosque," General Votel told reporters travelling with him in Qatar. "These aren't hastily made decisions."
Syrian state media and the Syrian Observatory for Human Rights, based in Britain, said last week that a series of strikes in Sousa near the Iraq border killed and wounded dozens; civilians as well as ISIS fighters.
The majority of those killed in the strikes were family members of IS fighters, Observatory chief Rami Abdel Rahman said on Sunday.
"Only six are Syrians, the others are mostly Iraqi," he said.
He said "12 Daesh terrorists" were killed in the raid, using an Arabic acronym to refer to ISIS.
"If there are credible claims of possible civilian casualties, they will be investigated," he added.
According to the Observatory, Thursday's strike killed 18 civilians, including seven children, all of whom were related to ISIS militants.
It said 11 ISIS fighters were also killed in the raid.
The coalition's statement, focused on the mosque, said monitoring of the building "made us aware" of when only ISIS fighters were present. It said the strike took place on Thursday when it was being used to coordinate attacks on the coalition and on US-backed Syrian Democratic Forces.
Sousa is in the last ISIS-held pocket in Syria and Syrian Democratic Forces have been on the offensive for weeks trying to clear the area of the extremists.
The statement did not acknowledge or refute the reports of civilian deaths from coalition air strikes in the area. "We conduct inquiries into all credible allegations of civilian casualties," it said.
ISIS fighters recently stormed a settlement for displaced people in the area and kidnapped 130 families.
ISIS overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" across the land it controlled.
But the militant group has since lost most of it to various offensives in both countries.
In Syria, the group has seen its presence reduced to parts of the vast Badia desert and a pocket that includes Sousa in the eastern province of Deir Ezzor along the border with Iraq.
Since 2014 the US-led coalition has acknowledged direct responsibility for more than 1,100 civilian deaths in Syria and Iraq, but rights groups put the number killed much higher.
The Syrian Observatory for Human Rights says coalition strikes in Syria alone have killed more than 3,300 civilians.
Syria's war has killed more than 360,000 people since it erupted in 2011 with the brutal repression of anti-government protests.
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