Kabul says the Afghan troops returned fire with small arms.
Afghan Defense Ministry spokesman General Zahir Azimi says the one-hour clash was in the Barmal district of Afghanistan's Paktika Province -- close to the town of Shkin.
The U.S. military has a strategic forward-operations base nearby, on the Afghan side of the border. The area is immediately adjacent to Pakistan's tribal region of South Waziristan.
Pashtun tribal fighters on Pakistan's side of the border there have been fighting Al-Qaeda-linked militants from Chechnya and Uzbekistan in recent weeks, trying to force the foreign militants out of their tribal region.
The Durand Line Issue
Paktika Governor Mohammad Akram Khpalwak told RFE/RL's Radio Free Afghanistan today that the fence being erected by Pakistan represented a seizure of Afghan territory.
"First of all, we cannot accept the line," Khpalwak said. "It is not demarcated and not clear where the border is. So [the Durand Line] is the basic issue. For solving the Durand Line issue, the problems of [ethnic-Pashtun] tribes living on both sides should be considered and they must be consulted. So it is very complicated issue and must be determined by the [ethnic-Pashtun] tribes living on both side of the line."
Islamabad says the fence was being erected on the border to prevent Taliban militants from crossing back and forth between the two countries.
Pakistan's Major General Wahid Arshad says the Afghan forces launched on unprovoked attack on Pakistani troops without entering Pakistan's territory.
Pakistani authorities recently announced they would fence 35 kilometers of selected areas along the porous 2,500-kilometer border to stop cross-border movement by militants. That announcement came in the wake of accusations by the Afghan and U.S. governments that Taliban insurgents regularly cross the border to attack Afghan and international forces.
Afghan President Hamid Karzai has argued that a fence would divide Pashtun tribes spread out along both sides of the Durand Line.
The Durand Line was created in 1893 by British officials as a defensive frontier for what was then British colonial India. It also served as a confidence-building measure between the British and tsarist Russian empires in the 19th century.
Pakistan inherited the Durand Line as its border when it became an independent country through the partition of British colonial India in 1947.
Afghanistan has never officially recognized the Durand Line as its border.
(RFE/RL's Radio Free Afghanistan contributed to the report)
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A Pakistani tribesman patrols near Wana in South Waziristan (AFP)
NO MAN'S LAND. Fighting erupted in March involving tribesmen in Pakistan's fiercely independent western regions, where reports suggest locally backed offensives targetted Uzbek and other foreign Taliban sympathizers.