A top Al Qaeda activist and suspected mastermind behind the September 11 attacks, Khalid Sheikh Mohammed, was held in a joint operation by the Federal Bureau of Investigation and Pakistan police and handed over to American authorities on Saturday.

Sheikh Mohammed, 37, for whose arrest the US had announced a $25 million reward, was captured along with two others in Rawalpindi and taken to an undisclosed location.

He is the third most important Al Qaeda operative after Osama bin Laden and Ayman al-Zawahari.

Sheikh Mohammed, a Kuwait-born naturalised Pakistani citizen, is alleged to be behind many Al Qaeda attacks.

The others who were arrested were an unidentified Arab and a Pakistani identified as Ahmed Abdul Qadoos, 42, of the Jamaat-e-Islami.

The Pakistani was not handed over to the Americans.

"This is a great success today, but the war on terrorism goes on tomorrow," Jim Wilkinson, a spokesman at US Central Command in Tampa, Florida, said adding, "there's still a lot of work to do."

Sheikh Mohammed, who allegedly organised the 9/11 attacks on the World Trade Centre and the Pentagon, was not charged in the case.

In 1996, a US court had indicted him in connection with conspiracies in the Philippines to bomb transpacific airliners and crash a plane into Central Intelligence Agency headquarters.

He was also allegedly behind April's bombing of a synagogue in Tunisia, which killed at least 19 tourists.

He is the uncle of Ramzi Yousef, who plotted the attack on the WTC in 1993. One of his brothers is also an Al Qaeda activist. Another brother died in Pakistan when a bomb he was making exploded.

Sheikh Mohammed, fluent in Arabic, English, Urdu and Baluchi, is believed to be close to Saad, bin Laden's son.

The North Carolina Agricultural and Technical State University, US, graduate had narrowly escaped arrest about a week ago in the southwestern Pakistani town of Quetta, official sources said.

Following his arrest in Rawalpindi, CIA Director George Tenet called US National Security Adviser Condoleezza Rice, who was staying at Camp David with President George Bush and First Lady Laura, to break the news. Rice then conveyed the news to the president.

Pakistan President Pervez Musharraf's spokesman, Rashid Qureshi, said, "It is a big achievement. He is the kingpin of [the] Al Qaeda."

"It's hard to overstate how significant this is," White House spokesman Ari Fleischer said. "It's a wonderful blow to inflict on [the] Al Qaeda."

PTI