Overshadowed by the sudden rise of the jihadist Islamic State, the al Qaeda terrorist organisation has sprung back to life with the release of an audio recording of its leader calling on Muslims everywhere to launch isolated attacks on the United States and its fellow "crusader" allies.

Al Qaeda supremo Ayman al-Zawahiri also acknowledged the rift between his group and ISIS but said his group is ready to unite with ISIS in fighting their common enemies.

"I call on all Muslims who can harm the countries of the crusader coalition not to hesitate. We must now focus on moving the war to the heart of the homes and cities of the crusader West and specifically America," said Zawahiri, an Egyptian militant leader who took over from Osama bin Laden who was killed during a US commando raid in Pakistan in May 2011.

The audio recording was released days after the US commemorated the 14^th anniversary of the Sept. 11 terrorist attacks launched by al Qaeda on the Twin Towers in New York City and three other US targets, killing nearly 3,000 people.

Zawahiri said young Muslims in the West should take after the Tsarnaev and Kouachi brothers, who perpertuated the Boston marathon bombings and Charlie Hebdo shootings in Paris, respectively.

The recording suggests that it was at least two months old as there was reference to former Taliban leader Mullah Mohamed Omar as still being alive.

At the same time, Zawahiri distanced himself from ISIS, calling the jihadist group "illegitimate," according to the Gospel Herald.

"We have endured a lot of harm from Abu Bakr al-Baghdadi (leader of ISIS) and his brothers, and we preferred to respond with as little as possible, out of our concern to extinguish the fire of sedition," he said.

"But Abu Bakr al-Baghdadi and his brothers did not leave us a choice, for they have demanded that all the mujahideen reject their confirmed pledges of allegiance, and to pledge allegiance to them for what they claim of a caliphate," Zawahiri said.

US intelligence is still reviewing the audio recording.

However, Zawahiri said that despite its differences with ISIS, al Qaeda needs to join forces with Baghdadi's group against the West and the coalition forces in Iraq and Syria.

"If I were in Iraq or Syria I would cooperate with them in killing the crusaders and secularists and Shi'ites even though I don't recognise the legitimacy of their state, because the matter is bigger than that," he said.

Commenting on the differences between al Qaeda and ISIS, former US National Counterterrorism Center director Matthew Olsen described it as "pretty interesting," ABC News said.

"Zawahiri until now has not been willing to openly condemn Baghdadi and ISIS. It highlights how deep the division is between al Qaeda leadership and ISIS," he said.