PAKISTAN HAS accused NATO helicopters of firing on two army checkpoints in the northwest, killing 25 soldiers.

Authorities have retaliated by closing a key border crossing used by the coalition to supply its troops in neighbouring Afghanistan.

Last night’s incident was a major blow to already strained relations between Islamabad and US-led forces fighting in Afghanistan. It will add to perceptions in Pakistan that the American presence in the region is malevolent, and to resentment toward the weak government in Islamabad for co-operating with Washington.

It comes a little over a year after a similar but less deadly incident, in which US helicopters accidentally killed two Pakistani soldiers near the Afghan border, whom the pilots mistook for insurgents. Pakistan responded by closing the Torkham border crossing to NATO supplies – as it did Saturday – for 10 days until the US apologised.

In a statement sent to reporters, the Pakistan military blamed NATO for Friday’s attack in the Mohmand tribal area, saying the helicopters “carried out unprovoked and indiscriminate firing.”

NATO officials in Kabul said this morning that they were aware of the reports, and would release more information after they were able to gather more facts about what happened.

Much of the violence in Afghanistan against Afghan, NATO and US troops is carried out by insurgents that are based just across the border in Pakistan. Coalition forces are not allowed to cross the frontier to attack the militants, which sometimes fire artillery and rockets across the line.

American officials have repeatedly accused Pakistani forces of supporting – or turning a blind eye – to militants using its territory for cross-border attacks. The border issue is the major source of tension between Islamabad and Washington, which wants to stabilise Afghanistan and withdraw its combat troops there by the end of 2014.