KABUL (Reuters/AFP) - Four hundred US marines staged a helicopter assault in the mountains of Helmand province in southern Afghanistan on Wednesday, part of efforts to secure Taliban-held areas, eight days before a presidential election. The 10,000-strong US Marine Expeditionary Brigade (MEB) in Helmand is the biggest element of a wave of reinforcements sent this year by President Barack Obama in an effort to turn the tide in an eight-year-old war commanders have described as stalemated. Operation Eastern Resolve II deployed 400 US troops and 100 Afghan soldiers to a Taliban stronghold of Helmand province, said Brigadier Gen Larry Nicholson, commander of the Marine Expeditionary Brigade in Afghanistan. In a statement from Camp Leatherneck, in central Helmand, he said the aim of the mission in Naw Zad district was to prevent Taliban fighters from acting on threats to disrupt presidential and provincial council elections next week. On August 20, 17 million Afghans are due to elect a president for the second time in history, but Taliban threats to block roads to polling booths and widespread fears of suicide attacks have clouded preparations. Our mission is to support the Independent Election Commission and Afghan national security forces, Nicholson said. Helmand is one of the worlds main poppy-producing regions and a route for Taliban fighters crossing the border from Pakistan to join the insurgency. US and Nato-led forces have been operating in the provinces centre and south in recent months in an effort to push out Taliban forces and secure populated areas of the vast region ahead of the elections. Thousands of US Marines who have poured into Taliban-controlled regions as part of a sweeping new war strategy under US President Barack Obama aimed at quelling the insurgency have come under intense attack from Taliban forces. Most deaths are caused by roadside improvised explosive devices, or IEDs, that rip through armoured vehicles and soldiers protective wear. The bombs are most often directed at security forces, accounting for most of their casualties, which reached a record level last month with 76 dead, but kill more civilians. Two journalists from the US-based global news agency Associated Press were seriously wounded in an IED attack while on patrol with US troops in southern Kandahar province on Tuesday, said the media group and military. Like Helmand, Kandahar is one of the main battlefields in the international fight against the Taliban since the 2001 US-led invasion. Defence ministry spokesman General Mohammad Azimi told reporters that nine districts remained under Taliban control - four in Helmand, two in Kandahar province, one in Uruzgan province and two in an unspecified region. Election officials concede that poor security could see voting suspended in a small proportion of the countrys 7,000-odd districts. Azimi said troops were deploying nationwide in an effort to provide security on polling day amid fears of suicide attacks. Election officials say that 300,000 Afghan forces and foreign troops will be deployed at polling booths. Our troops are in the process of being placed at polling stations as part of efforts to provide security for the elections. We are still hoping that more areas will be cleared before election day, Azimi said.