Four Afghan policemen have been killed when a roadside bomb ripped through their patrol vehicle in Afghanistan’s northern province of Faryab.Local officials said on Saturday that the blast happened in the Dawlat Abad district of the province, situated about 450 kilometers (270 miles) northwest of the capital Kabul, late on Thursday. The authorities said the remote-controlled bomb was detonated as the policemen were travelling on the Faryab-Jawzjan highway. They were carrying logistics for police checkpoints in the area, when the attack took place. Syed Abdul Salam Nizhat, the governor of Dawlat Abad district, said a police commander was among the dead. A security official, speaking on the condition of anonymity, identified the slain commander as Khalid Mohammad, adding that he had recently taken part in cleanup operations in the Qaysar and Pashtun Kot districts of Faryab province. No militant group has so far claimed responsibility for the attack. Roadside bombs and improvised explosive devices (IEDs) are by far the most lethal weapons Taliban militants use against Afghan forces, foreign troops, and civilians. The United States and its allies invaded Afghanistan as part of Washington’s so-called war on terror. The offensive removed the Taliban from power, but the country is still gripped by insecurity. In late April, the Taliban announced the start of their annual “offensive” against US-led and Afghan forces, vowing a new wave of attacks across Afghanistan. The militant group said it would use "every possible tactic" to inflict casualties on Afghan and US-led forces, specifically mentioning insider attacks and bomb attacks. The announcement prompted the Afghan authorities to beef up security in major cities across the country, including Kabul.