Four UN peacekeepers killed by a roadside bomb in central Mali on Wednesday were from Bangladesh, as were four peacekeepers seriously injured in the blast, the United Nations said. The incident occurred in the central Mopti region, where attacks by militants have surged in recent months, although UN officials did not say who was responsible for the blast. Six Malian soldiers were killed nearby a day earlier when their vehicle struck a landmine. "The Secretary-General conveys his condolences to the Governments of Bangladesh and Mali and his profound sympathies to the families and loved ones of the victims," a spokesman for UN Secretary-General Antonio Guterres said in a statement. Attacks targeting UN peacekeepers may constitute war crimes under international law, Spokesman Stephane Dujarric said. Guterres demanded that the perpetrators be apprehended and prosecuted, he said. The more than 11,000-strong mission in Mali has become the most dangerous in the world for UN peacekeepers, which are routinely attacked by militants. The threat of groups related to al-Qaida and ISIS continues to spread through Mali. Bangladesh is the worlds second-biggest contributor to UN peacekeeping operations with some 7,000 uniformed personnel deployed to missions around the world.