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Al-Qaeda’s notorious former leader, Osama bin Laden, expressed concern at the Islamic State’s violent tactics months before his death in 2011, according to a new batch of documents released by the US secret services on Thursday.

Bin Laden fretted about al-Qaeda in Iraq, which has since risen as the newly-branded Islamic State, suggesting the group’s impatience and violent approach deters Muslims from sympathising with their cause.

The comments were allegedly made in a series of letters sent by the militant to his family members and affiliates while he was in hiding in Pakistan.

The latest release from the trove of documents allegedly found when Navy Seals stormed the al-Qaeda chief's secret Pakistan compound and killed him in 2011, show bin Laden trying to keep his militant followers around the world aligned in his war against the United States.

The letters, some of which were written by Bin Laden and others on his behalf, also reveal concerns that his enemies could inject his sons with electronic tracking chips, while advising affiliates in Yemen to decrease the pace towards establishing an 'Islamic state'.

One letter to AQAP founder Nasir al-Wuhayshi warns not to move too fast against the Yemeni government because conditions were not yet right anywhere to form an Islamic state that could govern effectively and resist attacks from outside.

"Blood should not be shed unless we have evidence to show that the elements of success to establish the Islamic State and maintaining it are available or if achieving such goals is worthy of shedding such blood," he wrote.

"There might be a huge reaction that could drag us into a real war."

Focused on the US

The documents - which appear to date mostly from around 2010 - show the al-Qaeda chief determined to keep his group's focus on the United States as its prioritised enemy.

"The Ummah's enemies today are like a wicked tree," he wrote, using the Arabic term for the global community of Muslims. "The trunk of this tree is the United States."

The letters also reveal that Anwar al-Awlaki, a US-born Al-Qaeda cleric in Yemen, was a candidate to be named emir or chief of AQAP, with bin Laden asking for more biographical detail about him.

Despite this, Bin Laden noted doubt, stating "here we trust the people after we send them to the front line and test them."

Awlaki, whose writings inspired numerous converts to the militant’s cause, was killed by a US drone strike in September 2011.

A letter written by one of his aides suggests Bin Laden's rising frustration over his organisation nearly a decade after the shock 9/11 attacks on the United States.

Bin Laden "talked about the fear of our organisation aging, and reaching decrepitude like other organisations," the letter said.