**Four US soldiers have been killed in a roadside explosion in southern Afghanistan, Nato-led forces said.**The soldiers died after an improvised explosive device went off on Sunday, a statement by Nato’s International Security Assistance Force (Isaf) said.
Also on Sunday, a British soldier was killed in a separate blast in Helmand province, officials said.
Violence in Afghanistan has reached its highest level since the Taliban regime was toppled in 2001.