A suicide bomber driving a fuel tank blew himself up on Monday at a checkpoint run by opposition group Ahrar al Sham in the northern Syrian city of Aleppo, killing at least 23 people, a monitoring group said.
The Syrian Observatory for Human Rights also said that four of the group's commanders and four civilians are believed to be among those killed.
It said that the attack in the city's Sukari neighbourhood destroyed three nearby buildings and wounded dozens of people while many are believed to be stuck under rubble. Ahrar al Sham officials were not immediately available for comment.
In addition, a Russian air strike killed 12 people, including a local commander of the group in the northwestern province of Idlib, the monitoring group said. It was not immediately clear where the warplane struck. The Observatory said it targeted the commander's house, but rebel sources said it struck an office of the group.
This incident comes as the peace talks which are supposed to be held in Geneva on Monday to end the four year war were held up by a dispute over who should represent the Syrian opposition.
UN envoy Staffan de Mistura said he is still preparing the list which will be issued on Tuesday.
The UN estimates the death toll in Syria since the war started as an uprising to be at least 260,000, while the SOHR states that the number has now reached 350,000.
A further estimated 350,000 refugees have sought asylum in European Union countries since the war began in Syria in 2011.
About 4 million others have taken refuge in the neighbouring countries of Turkey, Lebanon, Iraq and Egypt, with Turkey hosting the largest group at over 2 million.