LASHKAR GAH, Afghanistan (Reuters) - An airstrike by the Afghan Air Force on a checkpoint in Gereshk district in the southern province of Helmand on Sunday killed around 10 members of the security forces and wounded nine others, officials said. The strike hit members of a special militia unit known as Sangoryan, who wear local clothes to blend into areas where the Taliban are active. Omar Zwak, a spokesman for the provincial governor, confirmed the incident but gave no details. The incident is the latest in a series of so-called  friendly fire  accidents in Helmand, the most recent in July when a U.S. airstrike killed a number of local police members during an operation in Gereshk district. In another incident, in June, at least three members of the Afghan Border Police were killed when a U.S. military aircraft opened fire on them during an operation in southern Afghanistan. Large parts of Helmand, Afghanistan s major opium-producing province, are in the hands of the Taliban. U.S. and Afghan commanders see air power as a vital weapon to prevent the area falling completely to the insurgents. Building up Afghanistan s own air power is a central part of President Ashraf Ghani s four-year strategic plan and the Afghan Air Force has steadily bolstered its capacity to provide logistical support to ground forces as well as air strikes. But one consequence has been an increase in the number of unintended casualties caused by accidents. According to U.N. figures published in July, there was a 43 percent increase in the number of civilian casualties caused by U.S. and Afghan airstrikes in the first half of the year. 