ISTANBUL — A Syrian journalist and filmmaker who documented atrocities by the Islamic State in Syria and trained hundreds of citizen journalists has been killed in a Turkish town near the Syrian border, rights activists said Monday, calling the death an assassination.

The killing of the journalist, Naji Jerf, in Gaziantep, Turkey, happened Sunday, one day before he and his family were scheduled to fly to France, where they were seeking asylum. Unconfirmed news reports from Gaziantep said he had been shot to death.

Mr. Jerf recently posted on YouTube a documentary on the killing of Syrian activists during the Islamic State’s occupation of the Syrian city Aleppo in 2013 and 2014. It was recently broadcast by the television network Al Arabiya.

His death is the latest in a string of killings of activists and observers who have drawn attention to human rights abuses during the nearly five-year civil war in Syria.