A Canadian photojournalist, Ali Mustafa, has been killed in the northern Syrian city of Aleppo, activists said. An official in the Aleppo opposition provincial capital said Mustafa, a freelancer who sold pictures to the SIPA and EPA agencies, was killed in the city on Sunday. The opposition Aleppo Media Centre also reported the death. "Four members of the (opposition) civil defence in Aleppo and 10 other civilians were killed, and dozens of others wounded, in a barrel bomb attack launched by helicopters against the Haydariyeh roundabout in the east of Aleppo city on Sunday morning," the centre said. "A Canadian journalist, Ali Mustafa was also killed... as he tried to document the massacre." Canada's foreign ministry said it was aware of reports that a Canadian national had been killed in Syria. Mustafa's sister, Justina Rosa Botelho, confirmed his death on her Facebook page, mourning her younger sibling. "Hope the world will understand what an angel my brother was. He cared more for others than himself," she wrote. Mustafa had worked in Syria periodically since early 2013, as well as in Israel and Palestine, Egypt and Brazil. In an interview last year about his decision to cover Syria, the world's most dangerous conflict for journalists, he said he "could not ignore this ongoing human tragedy". "Syrian people feel abandoned by the world. They are asking for our solidarity," he said. According to Reporters Without Borders, at least 27 professional journalists have been killed while covering the war in Syria. Dozens more have been kidnapped or jailed. The conflict has killed more than 140,000 people since March 2011.