Osama bin Laden kept pressing followers to find new ways to strike at the US while he was in hiding and his terror organisation was becoming battered and fragmented, officials have said, citing his private journal and other documents recovered in last week's raid.

Bin Laden suggested that they attack smaller cities and target trains as well as planes. Above all, he urged them to kill as many Americans as possible in a single attack.

Though he was out of the public eye and al-Qaida seemed to be weakening, Bin Laden never yielded control of the worldwide organisation, US officials have said.

His personal, handwritten journal and large collection of computer files reveal his hand at work in every recent major al-Qaida threat, including plots in Europe last year that had travellers and embassies on high alert, two officials said.

Analysts are continuing to review the documents.

The information shatters the government's conventional thinking about Bin Laden, who had been regarded mostly an inspirational figurehead whose years in hiding made him too marginalised to maintain operational control of the organisation he founded.

Bin Laden was in reality communicating from his walled compound in Pakistan with al-Qaida's offshoots, including the Yemen branch that has emerged as the leading threat to the United States, the documents indicate.

Though there is no evidence yet that he was directly behind the attempted Christmas Day 2009 bombing of a Detroit-bound airliner or the nearly-successful attack on cargo planes heading for Chicago and Philadelphia, it is now clear that they bear some of Bin Laden's hallmarks.

Don't limit attacks to New York, he said in his writings. Consider other areas such as Los Angeles or smaller cities. Spread out the targets.

In one particularly macabre bit of mathematics, Bin Laden's writings show him musing over just how many Americans he must kill to force the US to withdraw from the Arab world. He concludes that small attacks had not been enough. He tells his disciples that only a body count of thousands, something on the scale of the 9/11 attacks, would shift US policy.

He also schemed about ways to sow political dissent in Washington and play political figures against one another, officials said.

The communications were in missives sent via plug-in computer storage devices called flash drives. The devices were ferried to Bin Laden's compound by couriers, a process that is slow but exceptionally difficult to track.

Intelligence officials have not identified any new planned targets or plots in their initial analysis of the 100 or so flash drives and five computers that an assault team of Navy Seals hauled away after killing Bin Laden.

Last week, the FBI and Homeland Security Department warned law enforcement officials nationwide to be on alert for possible attacks against trains, though officials said there was no specific plot.

Officials have not yet seen any indication that Bin Laden had the ability to co-ordinate timings of attacks across the various al-Qaida affiliates in Pakistan, Yemen, Algeria, Iraq and Somalia, and it is also unclear how much the affiliate groups relied on his guidance.

The Yemen group, for instance, has embraced the smaller-scale attacks that Bin Laden's writings indicate he regarded as unsuccessful. The Yemen branch had already surpassed his central operation as al-Qaida's leading fundraising, propaganda and operational arm.

Al-Qaida has not named Bin Laden's successor, but all indications point to his No2, Ayman al-Zawahri.

The question is whether Zawahri, or anyone, has the ability to keep so many disparate groups under the al-Qaida banner.

The groups in Somalia and Algeria, for instance, have very different goals focused on local grievances.

Without Bin Laden to serve as their shepherd, it is possible al-Qaida will further fragment.