Dozens of people died in two explosions on Wednesday morning in Syria's northern city Aleppo, the London-based opposition Syrian Observatory for Human Rights claims.
A Syrian government official confirmed 25 people died in the blasts, which also injured 70 people, Al Jazeera reports.
Aleppo has been the site of intense fighting between government and rebel forces for the last two months. It has a population of 2.5 million and is strategically important for the warring sides.
The conflict in Syria has been running since March 2011, claiming over 20,000 lives according to UN data.
The Syrian authorities claim they face well-armed fighters supported from outside the country.
The blasts come as Russia's FSB chief, Alexander Bortnikov, said his organization was currently paying particular attention to “incidents of the involvement of people from the North Caucasus in anti-government groupings in the Afghanistan-Pakistan area and Syria."