In an audio message released today Zawahiri warned Pakistanis that the US was striving to "break up this nuclear-capable country and transform it into tiny fragments, loyal to and dependent on the neo-crusaders".

"The only hope to save Pakistan from this disastrous fate is jihad," said Zawahiri who, along with Bin Laden, is believed to be sheltering in the tribal belt along the Afghan border. He called on Pakistanis to band together and form a "citadel of Islam" on the subcontinent.

The message echoes a widely believed conspiracy theory in Pakistan that Washington is orchestrating violent chaos so US troops can storm in and disable the country's nuclear arsenal, estimated to number between 60 and 100 warheads.

"Zawahiri has cleverly read the situation and hit a very sensitive point," said Amir Rana, a militancy analyst.

The message comes amid crumbling public support for al-Qaida. A poll conducted in May found that 82% of Pakistanis considered the group posed a "critical threat" to their country, up from 41% in late 2007.

Although the survey was commissioned by a US organisation, WorldPublicOpinion.org, most analysts agree that support for al-Qaida's brand of extremism is sliding in Pakistan.

Many Pakistanis once lauded Bin Laden as a Robin Hood-style figure who defied America. But growing numbers are repulsed by al-Qaida claims of responsibility for suicide bombings that have killed hundreds of people, such as attacks on the Marriott hotel and the Danish embassy in Islamabad last year.

Al-Qaida has also been hit by a swing in public opinion against their local allies and protectors, the Taliban, after a video was broadcast showing a young woman being flogged by a turbaned fighter, and an army operation in the Swat valley and surrounding districts that displaced more than 2 million people, some of whom have started to return home this week. Al-Qaida's room for manoeuvre in Pakistan has also been pinched by US drone attacks that have killed 10 senior militants, according to US officials. That success, however, has been mitigated by hundreds of civilian deaths and a Pakistani backlash.

Analysts agreed that Zawahiri had hit a sensitive spot by mentioning US designs on Pakistan's nuclear arsenal.

"It's a very subtle move," said Talat Masood, a retired army general and defence analyst. "They are saying, 'The Americans are coming after your nuclear weapons and we can protect them.'"

Such theories were "very pervasive and deep rooted" in Pakistani society and were often fuelled by rightwing commentators in the Urdu-language press and sections of the powerful security establishment, Masood said.

"I've heard senior people saying this, including retired diplomats and generals. It's a cause for concern, because it shows the low levels of trust [between Pakistan and America]," he said.

Rana said the statement would have a limited impact on public opinion, but would "raise the morale of militant groups fighting with the Taliban".