Boris Johnson has said he is appalled by the "targeted and heinous murders" of seven paramedics in Syria.
Gunmen stormed an office belonging to the Syrian Civil Defence group, more commonly known as the White Helmets, on Saturday.
Seven of its members were killed while the culprits also stole two vehicles and other equipment during the attack in the north western town of Sarmin.
Since 2013 the White Helmets have saved thousands of lives in Syria, rescuing people from bombings and missile strikes.
Foreign Secretary Mr Johnson said: "I am appalled by the targeted and heinous murders of heroic White Helmets volunteers who put their own safety at risk to save lives on all sides of the Syrian conflict.
"Their bravery and humanity is so inspiring and their deaths a huge loss for their families, Syria and the world."