The Afghan Taliban says it has captured dozens of Pakistani soldiers after attacking their checkpoint in a cross-border raid.
Pakistani security sources confirm some troops are missing.
The Taliban says it is holding up to 40 Pakistani troops after its raid in the Mohmand tribal area on Monday.
Afghan officials said eight soldiers had been handed over to the Pakistani consulate in Jalalabad, but Pakistan's army said it had no knowledge of this.
The BBC's M Ilyas Khan says that while attacks by the Taliban on border check posts are relatively routine, it is unusual for Pakistani soldiers to be held by the militants in Afghanistan.
A Taliban spokesman told the BBC that it was in fact holding Pakistani troops on both sides of the border after Monday's attack.
It said 30 soldiers were being held in Afghanistan and 10 in Pakistan.
The Taliban says it captured the soldiers after over-running the checkpoint.
Local officials in the Mohmand area confirmed to the BBC that about 40 soldiers were unaccounted for.
Pakistani security sources said on Monday an undisclosed number of troops were missing.
An Afghan army commander in Jalalabad told the BBC that 10 Pakistani soldiers had been handed over to the Pakistani consulate, although the Pakistani army said it was not aware of this.
Meanwhile, the Afghan Taliban have dismissed the findings of a report which says Pakistan's intelligence service had a direct role in supporting the insurgents.
In an e-mail sent to the BBC, the Taliban said the report was "baseless propaganda".
The report, compiled by a London School of Economics scholar, said Pakistani intelligence provided funding, training and sanctuaries to the Taliban on a much greater scale than previously thought.
"The Islamic Emirate considers this report of the London School of Economics as merely baseless propaganda," the letter said.
The Taliban have also denied reports that their fighters hanged a seven-year-old boy last week on charges of spying in Afghanistan's Helmand province.
"After a full investigation by the Islamic Emirate leadership, it became clear that no event of execution had taken place," a Taliban statement said.
The Taliban criticised journalists for misreporting the event.