The two civil defence workers, known as the White Helmets, were killed after Russian air strikes hit their ambulance in the town of Khan Sheikhoun in Idlib province, the Syrian Observatory for Human Rights said.

The rescue group said a "double-tap attack" by Russian warplanes had "targeted" rescue workers repeatedly as they were evacuating injured civilians from the town.

Five other volunteers were also wounded, the group said.

The latest attack came nearly one week after regime air strikes on an ambulance in the town of Maaret al-Numan killed three rescue workers.

"The world continues to fail to protect us and other humanitarian workers," the group said in a statement on social media.

Seven other civilians were killed Wednesday in a series of regime air strikes on several parts of the Idlib region, which is home to around three million people, the Britain-based war monitor said.

One more civilian succumbed to wounds sustained days ago during regime air strikes on Khan Sheikhoun, it said.

In the north of nearby Hama province, jihadist rocket fire killed one girl on Wednesday, state news agency SANA said.

Idlib and parts of neighbouring Hama, Aleppo and Latakia provinces were supposed to be protected by a buffer zone under an September agreement between Russia and Turkey.

But the region has come under increased bombardment by the regime and its Russian ally since former Al-Qaeda affiliate Hayat Tahrir al-Sham seized most of the province at the start of the year.

Violence spiked in April, leaving more than 470 civilians dead, according to the Observatory.

The flare-up has also displaced 330,000 others, according to the United Nations, sparking fears of one of the worst humanitarian disasters in Syria's eight-year conflict.

The conflict in Syria has killed more than 370,000 people and displaced millions since it erupted in 2011 with the brutal repression of protests against President Bashar al-Assad.