Three Pakistani soldiers were killed in a Nato helicopter attack near the Afghan border, Pakistani officials say.
Security officials said Nato helicopters entered Pakistani airspace and targeted a paramilitary checkpoint.
Pakistani officials have now stopped Nato supply trucks from crossing the border at Torkham.
A Nato official confirmed there was an attack, but that it believed it was in Afghan airspace. It said the incident was under investigation.
Last week Pakistan protested after Nato forces crossed the border and killed about 50 alleged militants.
A Pakistani official told AFP news agency that Thursday’s attack was “unprovoked”.
“Nato helicopters entered our airspace and targeted a paramilitary checkpost killing three soldiers and wounding three others,” the official said.
In a statement, Nato said it had carried out an attack on suspected insurgents near a coalition base in the border area of Dand Patan district, in Afghanistan’s Paktiya province.
However, it said that the helicopter did not cross into Pakistani airspace, and that it was working with Pakistan “to ascertain if the two events are linked”.