Al Qaeda's second-in-command Ayman al-Zawahri
Al Qaida\'s second-in-command Ayman al-Zawahiri in a 2011 video (Reuters)

Al-Qaida leader Ayman al-Zawahiri has called for small-scale attacks within United States borders to "bleed America economically".

In an audio speech released online on the anniversary of 9/11 attack, Zawahiri, who has headed the terrorist organisation since Osama bin Laden was killed in 2011, said America was not a "mythical power". Small-scale attacks by one or a few mujahedeen would weaken the economy by diverting funding into tighter security, he said.

"We should bleed America economically by provoking it to continue in its massive expenditure on its security, for the weak point of America is its economy, which has already begun to stagger due to the military and security expenditure," he said.

"As we defeated it in the gang warfare in Somalia, Yemen, Iraq and Afghanistan, so we should follow it with war on its own land. These disparate strikes can be done by one brother or a few of the brothers."

He did not rule out the possibility of a large strike on the US.

Western intelligence has long warned of the radicalisation of rogue individuals who seem to have no direct link to al-Qaida and might pose a greater risk to American security.

Zawahiri approved the bombing of the Boston Marathon in April, which was carried out by two ethnic Chechen Muslim brothers. The attacks killed three people and injured 264.

"The Boston incident confirms to the Americans that they are not facing individuals, organisations or groups, but they are facing an uprising in the Muslim community that rose in jihad to defend its soul, dignity and capabilities," he said.

"What the American regime refuses to admit is that al-Qaida is a message before it was an organisation."

In August the Obama administration temporarily shut down 19 embassies and consulates across the Middle East and North Africa after intercepting communications between al-Qaida leaders indicating possible strikes on US missions abroad.