A roadside bomb has killed three soldiers from the US-led coalition force in Afghanistan, the US military said. Violence has surged to its worst level this year, resulting so far in the killing of some 4,000 people, a third of them civilians, the bloodiest period since US-led troops overthrew Taliban's government in 2001. The roadside bomb hit a vehicle of the coalition soldiers on Wednesday evening in the western part of the country, the US military said in a statement late the same day. It did not give the nationality of the victims. Resurgent Taliban rely heavily on suicide and roadside bomb attacks in their attempt to topple the Western-backed government and to cause the withdrawal of foreign forces from the country.