A powerful roadside bomb explosion has killed at least four police officers in the troubled northern Afghanistan, security sources say.Local security officials say the officers guarding a regional mayor were killed when their convoy struck an improvised explosive device (IED) in Barak district of Badakhshan Province on Tuesday. At least one police officer was also wounded in the fatal blast. However, sources say the mayor has escaped the attack unhurt. No group has claimed responsibility for the attack. Provincial police forces say the bomb had been planted by the Taliban militants. Roadside bombs and improvised explosive devices are by far the most lethal weapons Taliban militants use against Afghan forces, foreign troops, and civilians. The Taliban recently announced the start of their annual "spring offensive" across Afghanistan and vowed to increase their attacks on US-led troops stationed in the country as well as Afghan forces. The US-led war in Afghanistan removed the Taliban from power, but insecurity continues to rise across Afghanistan, despite the presence of many US-led troops there. The United States and its allies invaded Afghanistan in 2001 as part of so-called war on terror. The offensive removed the Taliban from power, but insecurity remains across the country.