Pakistani and American officials have long been tightlipped on the status of Nasar, who has had a $5 million U.S. bounty on his head. He has been described by the U.S. Justice Department as a former trainer at

Osama bin Laden's camps in Afghanistan who helped teach extremists to use poisons and chemicals.

Another Pakistani official confirmed the Quetta arrest but had no information on Nasar's whereabouts.

"He had been interrogated by us. He had been interrogated by our American friends," said the official, who also declined to be identified because of the secretive nature of his activities. He added that both Syrian and U.S. authorities wanted to take Nasar into custody.

A picture and short biography of the red-haired Nasar was recently removed from the U.S. government's Rewards for Justice Web site. Justice and State Department officials declined to say why Nasar was no longer profiled.

It would not be the first time Pakistan ¡ª a key U.S. ally in the war against terrorism ¡ª has detained al-Qaida terrorists and turned them over to the Americans.

Pakistan says it has captured more than 750 al-Qaida suspects since the Sept. 11, 2001, attacks and has handed most of them to the United States.

They include al-Qaida's former No. 3, Khalid Shaikh Mohammed, a key planner of the Sept. 11 attacks, who was arrested in March 2003 during a raid near Islamabad, and his purported replacement, Abu Farraj al-Libbi, who was detained in May 2005 in Pakistan's northwest.

Media reports have linked Nasar, who holds Spanish citizenship, to the 2004 commuter train bombings in Madrid that killed 191 people, and to the July 7, 2005 attacks in London that left 56 dead, including the four bombers.

In September 2003, Nasar was among 35 people named in an indictment handed down by a Spanish magistrate for terrorist activities connected to al-Qaida. His exact role, if any, to either the Madrid or London bombings is unclear.

He is also wanted for a 1985 attack on a restaurant near a military base close to Madrid airport that left about 20 people dead ¡ª regarded as the first international Islamic terrorist attack to take place in Spain.

Spain's ambassador to Pakistan, Jose-Maria Robels, said that Spain had sought information from Pakistan about Nasar's reported arrest in November but had received no reply.

"Pakistan knows our interest but we have not had any official answer," he said in Islamabad on Tuesday.

Nasar, who lived in Spain and was married to a Spanish woman, also stayed in London during the mid-1990s before traveling to Afghanistan where he was believed to have been part of Bin Laden's network, a Western diplomat in Islamabad said.

His movements have been traced to Syria, Afghanistan, Iran, Iraq and at least two European capitals.