Pakistani troops fired heavy artillery at Afghan forces at their main Khyber Pass border crossing on Wednesday, the Pakistani military said, an escalation after days of clashes that have killed four people and stranded thousands on both sides.
Relations between the U.S. allies have never been close but have been strained over the past 15 years by Afghan accusations that Pakistan supports the Taliban who are fighting to unseat the U.S.-backed government in Kabul.
Pakistan denies that.
The countries have blamed each other for the fighting that broke out on Sunday at the main crossing point between them over the construction of a new border post on the Pakistani side.
"When our people began construction work on the gate on Wednesday, Afghan forces again opened fire at our troops and construction workers," said a Pakistani security official who declined to be identified.
He said Pakistan had retaliated with long-range artillery and mortars. There was no word on any casualties.
The Pakistani military spokesman's office confirmed the Wednesday fighting.
Afghan officials, however, denied any fighting on Wednesday, but said an Afghan border guard was killed and five were wounded overnight on Tuesday. It was not immediately clear if the officials were talking about the same incident.
Pakistan's Foreign Office summoned the Afghan ambassador to demand that Afghanistan put an end to the "unprovoked firing" and to protest against the death of a Pakistani army major, shot on the border in a clash this week.
The U.S. State Department on Tuesday urged "a calm resolution to the tension".
By Wednesday, trucks carrying goods to Afghanistan had been stranded on the Pakistani side of the border for four days. A Reuters journalist at the scene said trucks lined the road all the way from the northwestern city of Peshawar to the Torkham border.
Pakistan says the border gate it is building is well on its side of the border and will help stop militants from crossing and help fight drug trafficking.
Pakistan's foreign policy chief, Sartaj Aziz, expressed concern over what he called the Afghan army's attempts to "disrupt border management efforts" by Pakistan.
Afghanistan objects to all Pakistani construction on a 2,200-km (1,370-mile) border it says was unfairly imposed by British colonialists in the 19th century and which it has never recognized.
Relations between Pakistan and Afghanistan have been particularly strained in recent months over Afghan officials' anger at what they see as insincere efforts by Pakistan to help with peace talks between the Afghan government and the Taliban.