Pakistani security agencies are investigating whether one of two al-Qaeda suspects they have arrested is a Syrian linked to the July 7th London bombings.
He is also believed to be a key figure in Osama bin Laden's terror network in Europe.
The two suspects were captured during a raid on a house in Quetta, the capital of Pakistan's south-western Baluchistan province, said an intelligence official.
A government official confirmed the arrests and said authorities were investigating whether one of the suspects was Mustafa Setmarian Nasar, alleged to have had a key role in the March 11th, 2004, Madrid bombings that left 191 people dead and more than 1,500 people injured.
Nasar was born in Syria but also has Spanish nationality. His name has also been linked to the July 7th bombings in London that killed 52 people.
There was no information about the identity of the second suspect.
Pakistani government spokesmen, and the US embassy said they could not immediately confirm the arrests.