British troops have launched helicopter advances in Afghanistan's southern Helmand province to prepare the battlefield for a major NATO operation, the British military said on Friday. The British operations are on the outskirts of Marjah, a warren of desert canals held by the Taliban, which U.S. Marines say they intend to seize soon in what will be one of the biggest assaults of the eight-year-old war. British and Afghan troops were carrying out "shaping operations" in Helmand's Nad Ali district as part of an initial phase of Operation Moshtarak, or "together," a large assault which will seize the entire district, the British military said. Nad Ali includes Marjah, which the U.S. Marines describe as the last major Taliban-held bastion in the south of the province, Afghanistan's most violent region, which produces most of the country's illegal opium crop that helps fund the insurgency. The assault on Marjah itself will be the first operation to employ some of the 30,000 new reinforcements sent by U.S. President Barack Obama at the end of last year.