Three civilians, including two school pupils, were also killed when Afghan border guards and Pakistani soldiers exchanged fire, a governor said.

The fighting erupted in Afghanistan's Paktia province when Pakistani troops fired on Afghan border guards building a security post on the disputed frontier, provincial governor Rahmatullah Rahmat told AFP.

The Afghan troops returned fire, the governor said.

The fighting continued into the afternoon and the Pakistani troops were using heavy weapons including rockets, he said.

"Two school children were killed when a Pakistani rocket landed in their school, and a police officer and another civilian were also killed by Pakistani rockets," Rahmat said.

The clash was the second in less than a month. On April 19 security forces from both sides fought for several hours after Afghans tore down part of a fence being erected by Pakistan. No one was hurt.

Pakistan says it is putting up the fence to stop the cross-border movement of militants involved in the Taliban-led insurgency in Afghanistan.

An official said last week that it had completed 20 kilometres of the fence. Another 15-kilometre stretch of the border would soon be fenced, Pakistan military spokesman Major General Waheed Arshad told AFP.

The Pakistan army had also deployed extra troops and increased patrols in the area, which faces southeastern Afghanistan.

The Afghan government strongly objects to the fence, saying it will not help stop the rebels crossing into the country but will separate tribes on the both side of the frontier.

Afghanistan also does not accept the border, which it calls the Durand Line. The frontier was drawn up by colonial Britain in 1893.