US Ambassador Ryan Crocker and British High Commissioner Robert Brinkley were called to the Pakistani foreign office separately and a "strong protest" was lodged with coalition authorities, an official statement said.

"They were told that the coalition authorities should investigate the incident and ensure that such incidents do not occur in the future," the foreign office statement said.

The diplomats expressed "deep regrets and said that the incident was being investigated," it added.

The casualties occurred on Monday afternoon when coalition troops stationed on the Afghan side of the porous border fired upon one of Pakistan's checkpoints near Shawal, in the troubled North Waziristan tribal district.