The Syrian civil defense group known as the White Helmets said eight of their volunteers have been killed in an air strike on their office in rebel-held Hama Province.
White Helmets officials in Hama on April 29 said an air strike on their offices in the city of Kfar Zeita killed eight members. They said five bodies had been recovered from the rubble and rescue workers were searching for the others.
The U.K-based Syrian Observatory for Human Rights, an opposition monitoring group, said the death toll was likely to rise.
It was not immediately clear who carried out the air strikes. Syrian government and Russia aircraft have targeted opposition-held areas in the past, including Hama Province.
The government of President Bashar al-Assad is attempting to repulse a rebel offensive in the area.
The White Helmets rescue volunteers operate in opposition-held parts of Syria, usually helping to pull victims and survivors from bombed buildings and homes.
Syria's conflict began in 2011 as a civil war. Now, with international forces drawn into the conflict, there are several opposition groups and Muslim extremists who are fighting against Assad's forces and against each other.
At least 250,000 people have been killed in the conflict, which has caused the world's worst refugee crisis since World War II.
The United States and Turkey have supported various rebel groups, with Russia and Iran support Assad’s government.