Cross-border clashes broke out Monday between Afghan and Pakistani security forces for the second time in days, escalating tensions between Kabul and Islamabad, officials said.
Ties between the fractious neighbours have become increasingly strained despite renewed efforts last month from US Secretary of State John Kerry to get them to work more closely on peace efforts in Afghanistan.
Afghanistan and Pakistan are in dispute over a site where Pakistan has tried to construct a gate on what Afghan officials say is Afghan territory. Clashes last Wednesday in the same spot killed an Afghan guard and wounded two Pakistanis.
The porous border is unmarked in places and a key battleground in the fight against Taliban violence plaguing both countries.
"Pakistan shamelessly, again today, tried to construct those buildings. This action led to fighting," said Ahmad Zia Abdulzai, spokesman for the government in eastern Afghan province Nangarhar.
Abdulzai said the fighting involved heavy and light weapons, but refused to say who started it, when asked.
A senior Afghan official in Kabul confirmed the incident.
Pakistani officials blamed Afghans for starting the clashes.
"Afghan troops opened unprovoked fire from across the border at our post... They fired mortars and automatic weapons," one Pakistan official told AFP on condition of anonymity.
"Our troops responded with retaliatory fire. There have not been reports of any casualties so far. The exchange of fire continues at intervals," a second official said, also speaking on condition of anonymity.
Afghanistan and Pakistan are both US allies in its war on terror.
But Kabul accuses Islamabad of playing a double game in supporting Taliban insurgent attacks on US and Afghan troops. Pakistan denies the allegations and is locked in its own battle against Pakistani Taliban.
Pakistan, which backed Afghanistan's 1996-2001 Taliban regime, is seen by the West as having a central role in negotiating a political settlement with Taliban insurgents who shelter in Pakistan's border districts.