US president Barack Obama’s biggest international challenge on Thursday became more daunting still with the news that his ambassador to Afghanistan has deep reservations about the US military’s call for more troops for the country.

In a move that shows the split not just in Washington but between US efforts on the ground, Karl Eikenberry, the former commander of US forces in Afghanistan who now heads the embassy in Kabul, has told Mr Obama in writing of his opposition to fulfilling the military’s request unless the Afghan government first changes its ways.