WASHINGTON (AFP) - The United States expressed sadness and horror Sunday (Aug 13) over the killing of seven members of Syria's White Helmets rescue service in an extremist-held town near Idlib.

The seven were killed Saturday by unidentified assailants in a raid on their base in Sarmin, nine kilometres east of the city of Idlib.

The town is controlled by the Hayat Tahrir al-Sham jihadist alliance, which denounced what it called an "ugly crime". State Department spokeswoman Heather Nauert extended condolences to the families of the victims.

"We are saddened and horrified to hear about the brutal murders of seven Syrian Civil Defence Members, more widely known as the White Helmets," she said.

"These cowardly acts of masked men took the lives of civilian volunteers who work tirelessly as first responders in order to save lives in incredibly dangerous environments," she said.

The White Helmets emerged in 2013, working to rescue civilians in rebel-held areas.

They have since gained international renown for their daring rescues, often filmed and circulated on social media, and were nominated for the 2016 Nobel Peace Prize.