A Pakistani man accused of aiding al-Qaeda and the Taleban has been held in a joint US-Afghan operation, security sources have told the BBC.
The governor of Kunar Province in Afghanistan said that Haji Nadir was a "trusted al-Qaeda operative" and was in Afghanistan to carry out attacks.
Security sources said Mr Nadir was suspected of training fighters and making bombs for use in Afghanistan.
He has been taken to Bagram air base for questioning by American forces.
Mr Nadir was arrested on Thursday in Afghanistan's Kunar Province, after crossing the border from Pakistan.
Security sources said Mr Nadir, from Dir district in the North-West Frontier Province of Pakistan, had been actively involved in the increasing number of roadside bombings carried out by insurgents in the eastern Kunar Province.
There have been at least 10 roadside bombs this year, compared to 17 in 2005 and five in 2005.
US-led forces have frequently carried out operations in the border region, targeting senior al-Qaeda leaders suspected to be hiding in the area.
In January Pakistani President Pervez Musharraf claimed a "close relative" of al-Qaeda number two Ayman al-Zawahiri had been killed in one attack.
A number of local villagers were also killed during that operation.
Mr Nadir's arrest comes at a time of renewed diplomatic sparring between Pakistan and Afghanistan.
In an interview with the BBC, Afghan Foreign Minister Abdullah Abdullah dismissed Pakistan's claims that intelligence information passed to Pakistan was "outdated".
President Musharraf had said on Monday that information handed over by Afghan intelligence about Taleban leaders in Pakistan was "old and outdated", and Afghan President Hamid Karzai did not know what was happening on in his own country.
Up to 30 suspected militants were killed in an operation in Waziristan
Dr Abdullah said: "The information we provided included the whereabouts of Taleban leaders, the whereabouts of training camps for Taleban, the areas where Taleban [fighters] cross borders.
"We were sure of the value of the intelligence... and some of it was very solid evidence of what was happening."
The Pakistani army has been involved in renewed military action against suspected pro-Taleban militias in the tribal region of North Waziristan over the last week.
Reports say nearly 150 people have died in the fighting.
But Dr Abdullah repeated Afghanistan's demand for Pakistan to do more to stop militants crossing into Afghanistan.