"The civil defense center in Sarmin was the target of an armed attack by an assailants in which seven volunteers were killed", the White Helmets said in statement.
The attack occurred in the city of Sarmin in Idlib province, which is controlled by Hayat Tahrir Al-Sham, a coalition of Salafi jihadist groups that includes Fateh al-Sham.
Some reports have claimed one of the men killed in Saturday's attack is a volunteer who featured in one of the group's most moving rescues. The latter group had been known as Jabhat al-Nusra before renouncing its ties to al Qaeda.
The militant group said the attack was aimed at "undermining our revolution", adding that it stood ready to protect the White Helmets in "liberated areas" of Syria.
"Until now it is mostly likely a crime. It may likewise be an assault expected to hurt the picture of the Nusra Front and to demonstrate that Idlib is not protected", said Rami Abdurrahman who heads the Observatory.
The dissident, who asked that his name not be utilized inspired by a paranoid fear of backlashes, said IS sleeper cells have been found in Sarmin. Sarmin is in Idlib province, which witnessed clashes recently between rival insurgent groups.
They operate a rescue service in rebel-held parts of Syria, which have been subjected to fierce bombardment by the government and Russia's air force that has levelled whole city districts during the country's civil war.
They have since gained widespread worldwide renown for their daring rescues, with footage often taken and circulated on social media.
Although they work exclusively in rebel-held areas, they are non-partisan.
Elsewhere in Syria, a suicide attacker blew himself inside a training camp for the Army of Islam rebel group in the southern town of Naseeb, near the border with Jordan, killing more than 20 fighters.
Ahmad al-Masalmeh, a restriction extremist situated in the southern region of Daraa, said in regards to 80 Army of Islam individuals were eating inside a tent when the suicide assailant strolled in and exploded himself.
He said 30 were killed, 20 were wounded and six are still missing.