British troop numbers in Afghanistan could be scaled back in five years as the Afghan army is strengthened, though some will have to stay behind in a support role, the British army's new chief said on Thursday. General David Richards, who took over as chief of general staff in August, told the BBC he agreed with the analysis by the U.S. commander of international forces in Afghanistan, General Stanley McChrystal, that additional troops would be needed. Troops numbers could be reduced once the Afghan army and police were trained in sufficient numbers to take over more responsibility in the fight against Taliban insurgents, he said.