Pakistan forces have begun work on fencing and mining sections of border with Afghanistan, Governor of southern Paktika province Akram Khapalwak said.
The residents of the province, which borders Pakistan and where Taliban activity is rife, had reported to officials that some 2,000 Pakistani "militia" as well as regular forces were taking part in the fencing operation, he quoted by Afghan news agency as saying on Monday.
The authorities in Kabul declined comment on reports coming in from the border areas.
The controversial border operation was going on at Azam Warsak, Qamardin Karez and border areas in Tarwa and Gomal districts, the governor said.
He alleged that Taliban and Al-Qaeda elements were helping the Pakistani forces in erecting fences and pointing out areas where these might be put up.
The governor said the Pakistani side was active in areas indicated by the extremists and was leaving infiltration routes alone. "The consequences of this could be serious if the international community does not intervene," he added.
The Pajhwok Afghan News (PAN), an independent local news agency, said hundreds of people demonstration against the border fencing in Tarwa district on Monday.
The demonstrators were joined by tribal elders, ulema and students.
Earlier reports had suggested that Islamabad planned to begin implementing its decision - taken barely a week ago- as regards the border sometime this month. No one had expected fencing to begin quite so soon.
The subject is very sensitive in Afghanistan and had drawn sharp comment from President Hamid Karzai last Thursday at his joint press conference with visiting Pakistan Prime Minister Shaukat Aziz.