A US air strike killed at least two Afghan policemen and wounded four others in Helmand. Officials said today, in apparently the first "friendly fire" incident since American Marines returned to the southern province in April. <br/><br/>Afghan border police were on a patrol in the volatile district of Nad Ali when they came under fire during a military operation around midnight yesterday.<br/><br/>The deployment of some 300 Marines to the poppy-growing province came as the Trump administration seeks to craft a new strategy in Afghanistan.