A possible air strike is planned on the suspected al Qaeda target who is surrounded by troops in Pakistan, sources have said.
It is thought that the target could be Osama bin Laden's second in command, Ayman al-Zawahri.
Troops said they cornered the man thought to be the mastermind of the September 11 attacks on the US, as part of an operation near the Afghan border.
The operation, involving hundreds of troops and paramilitary rangers, has been carried out in the mountainous South Waziristan region.
The identity of the man surrounded has not been confirmed.
But President Pervez Musharraf said: "(Judging by) the resistance that is being offered by the people there, we feel that there may be a high value target."
Sky News' Foreign Affairs Editor Tim Marshall said: "Most people believe the number two is actually the brains (behind the al Qaeda network)."
And he added: "If it's true it is an enormous strike. It cannot be underestimated."
On Tuesday at least 41 people, including 15 soldiers and 26 suspected militants were killed in fighting in the area.
Army spokesman General Shaukat Sultan said there had been an unknown number of casualties in continuing action Thursday. Last Updated: 22:10 UK, Thursday March 18, 2004