Kuwaiti-born Khalid Shaikh Mohammed is already on the FBI's most wanted list for his alleged part in a plot to bomb American airliners flying to the US from south-east Asia in January 1995.

He's the primary brains of the plot. He's also a major player in the al-Qaeda organisation

US official

Now senior US officials say the FBI has identified him as one of al-Qaeda leader Osama Bin Laden's key lieutenants, and say that he took a central role in planning the attacks.

"There's lots of links that tie him to 9-11. He's the most significant operational player out there right now," one counter-terrorism official said, speaking on condition of anonymity.

Primary planner

The official said that although many of Bin Laden's associates were involved with organising the attacks there was growing evidence that Mr Mohammed played a central role.

Mohammed is said to be a top al-Qaeda member

"He's the primary brains of the plot. He's also a major player in the al-Qaeda organisation," the official said.

"He planned this whole operation. We would like to locate, arrest him and bring him to justice," he added.

Mr Mohammed is currently believed to be in either Afghanistan or Pakistan.

The evidence against Mr Mohammed includes the fact that he transferred money that was used to pay for the attacks, the officials said.

Al-Qaeda lieutenants are said to provide money and training to the terrorists who carry out their ground operations as well as picking the attack targets and dates.

"In terms of al-Qaeda bad guys, he's in the top half dozen or so," one official said.

Bomb plot

Mr Mohammed has not been charged in connection with the 11 September attacks which killed more that 3,000 people.

He was indicted in the United States in 1996 for his alleged role in a plot to blow up US civilian airliners over the Pacific and the US government is offering a reward of $25m for his capture.

My guess is, if he (Bin Laden) were active, we would know it. We would have some visible sense of it, which we haven't seem to have had, for some reason

Donald Rumsfeld

One of the US officials said Mr Mohammed is a relative of Ramzi Ahmed Yousef, currently serving a life sentence in Colorado for the 1993 World Trade Center bombing.

Yousef was also convicted for his role in the plot to bomb airliners over the Pacific, which was foiled when Philippines authorities arrested one of Yousef's associates in Manila.

The plot was hatched by Islamic extremists linked to al-Qaeda.

Bin Laden activity

The officials said Mr Mohammed is also a close associate of Abu Zubaydah - said to be a member of Bin Laden's inner circle and one of al-Qaeda's most senior operators.

Abu Zubaydah is now being quizzed by the US after being captured in Pakistan in April.

The US has repeatedly warned that Bin Laden's top aides are as dangerous as the leader himself and that capturing them is a priority.

Meanwhile US Defence Secretary Donald Rumsfeld said Bin Laden does not seem to be formally directing al-Qaeda at the moment.

"My guess is, if he were active, we would know it. We would have some visible sense of it, which we haven't seem to have had, for some reason," Mr Rumsfeld said in an interview with the Washington Post published on Tuesday.

Mr Rumsfeld said he did not know whether Bin Laden was laying low for security reasons, suffering from an illness, or dead.