The US Defence Secretary, Leon Panetta, has warned that the United States is reaching the limits of its patience with Pakistan because of the safe havens the country offered to insurgents in neighbouring Afghanistan.
It was some of the strongest language used by a top US official to describe the strained ties between Washington and Islamabad.
"It is difficult to achieve peace in Afghanistan as long as there is a safe haven for terrorists in Pakistan," said Mr Panetta, speaking in the Afghan capital Kabul where he held talks with military leaders amid rising violence in the war against the Taliban and a spate of deadly attacks, including a Nato air strike said to have killed 18 villagers.
"It is very important for Pakistan to take steps. It is an increasing concern, the issue of safe haven, and we are reaching the limits of our patience," he told reporters.
The US has long pushed Pakistan to do more to help in the war against militancy, but the relationship has received a series of blows, not least by a US raid to kill Osama bin Laden last year that humiliated Islamabad.