Al Qaeda leader Ayman al-Zawahri has called on young Muslim men in the United States and other Western countries to carry out lone wolf attacks and urged greater unity between militants.

“I call on all Muslims who can harm the countries of the crusader coalition not to hesitate,” he said in an audio recording posted online, referring to nations making up the Western-led coalition in Iraq and Syria, which include Australia.

“We must now focus on moving the war to the heart of the homes and cities of the crusader West and specifically America.”

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He suggested Muslim youth in the West take the Tsarnaev brothers, who carried out the Boston marathon bombings, and the Kouachi brothers, responsible for the Charlie Hebdo shootings in Paris, as examples to follow.

It was not clear when the recording was made but references to former Taliban leader Mullah Mohamed Omar as being alive suggest it is at least two months old.

Omar’s death was announced by Afghanistan’s government in late July.

Zawahri reiterated his position on Islamic State, repeating what he said in a recording posted on Wednesday, that he viewed the group’s claim to be a caliphate as illegitimate but would join them in fighting Western and secular forces in Iraq and Syria.

The former Egyptian doctor urged unity between Islamist militant factions in Syria and Iraq, where a Western-led coalition is bombing Islamic State targets, but recognised it would be difficult.

He called for the formation of an independent sharia court to settle disputes.