Syria regime bombing kills 16 civilians in Idlib
It comes as fighting resumed in Idlib, where dozens have been killed on the edge of the opposition province, home to around 3 million civilians.
The UN has warned that the spike in violence could spark one of the worst humanitarian disasters in Syria's eight-year war.
A regime or Russian airstrike hit an ambulance in the town of Maaret al-Numan, killing three rescue workers, the Syrian Observatory for Human Rights said.
Syrian charity Benefsej which owned the ambulance said three of its workers had been killed.
"It was a direct targeting of the ambulance," said charity member Fouad Issa.
A woman also died in the ambulance while she was being transported for treatment, he added.
In a statement posted on social media networks, the charity described the attack as a "violation of international humanitarian law".
"Ensuring the protection of humanitarian and health facilities via the international community is a necessity for us to continue our work".
Russia and rebel backer Turkey brokered an agreement in September seeking to stave off an all-out regime assault on Idlib but intense regime and Russian bombing since April has killed 440 civilians.
The UN says more than 23 hospitals and one ambulance have been hit in the Idlib region since late April, while 330,000 people have been forced to flee their homes.