KABUL  - Afghanistan and the United States have started talks that will eventually define how many American troops stay in the country after most Nato combat forces leave at the end of 2014, and the scope of their mission.The bilateral security negotiations could take months, and are expected to be difficult. The round of talks that began on Thursday will cover the legal basis for US soldiers to work in Afghanistan post-2014.“This document is intended to provide the legal authority for US armed forces and their civilian component to continue a presence in Afghanistan with the full approval of the government of Afghanistan,” said James B Warlick, deputy special representative for Afghanistan and Pakistan, who will be leading the US delegation.The thorniest issue in future talks will be whether US soldiers in Afghanistan are given immunity from prosecution under Afghan law.President Hamid Karzai has long demanded that US soldiers be answerable to Afghan law, but the United States insists that its soldiers accused of crimes in Afghanistan be tried in America.Underscoring the likely difficulty of the talks, US military prosecutors are seeking the death penalty for Army Staff Sergeant Robert Bales, accused of killing 16 Afghan villagers in a March massacre. The Afghan government wants Bales to be publicly tried in Afghanistan.“Afghanistan wants a strategic pact with US but will seriously consider the red lines,” said Aimal Faizi, Karzai’s chief spokesman.“The negotiations between the two countries are due to start today and the most important issue for Afghanistan is its national sovereignty and national interest,” Faizi told Reuters.Eklil Hakimi, Afghanistan’s ambassador to the United States, leads the Afghan team. This year, the two countries signed a strategic agreement which provides a framework for a post-2014 US role in Afghanistan, including aid assistance and governance, but not troop numbers.