Ayman al-Zawahiri, the leader of al-Qaida, has called on Muslims around the world to support rebels in Syria who are seeking to overthrow Bashar al-Assad.

The statement is the most explicit attempt yet by the terrorist group to intervene in the ongoing Syrian conflict.

In the eight-minute video titled Onwards, Lions of Syria, posted on extremist websites on Saturday, Zawahiri calls on Muslims in Iraq, Jordan, Lebanon and Turkey to join the uprising against Assad's "pernicious, cancerous regime", and warned Syrian rebels not to rely on the west for help.

"Wounded Syria still bleeds day after day while the butcher, son of the butcher Bashar bin Hafiz [Hafez al-Assad], is not deterred to stop," Zawahiri said. "But the resistance of our people in Syria despite all the pain, sacrifice and bloodshed escalates and grows."

Al-Qaida, seriously weakened by the loss of its leader Osama bin Laden last year, has played no significant role in the ongoing unrest associated with the Arab spring.

However, the group has made persistent attempts to indirectly influence those opposing autocratic regimes across the Middle East, and to intervene directly. Late last year senior militants linked to the group travelled from Afghanistan to Libya in an effort to boost recruitment in the chaotic aftermath of the fall of Muammar Gaddafi.

The Assad regime has repeatedly said Islamic militants are behind the violence in Syria, a claim rejected by opposition groups who say it is designed to discredit them in the eyes of the international community.

At the weekend, US newspapers cited American officials blaming al-Qaida in Iraq, a largely autonomous affiliate of the main group, for two recent bombings in the Syrian capital, Damascus, and a suicide attack in Aleppo on Friday that killed at least 28 people.

Zawahiri said: "If we want freedom, we must be liberated from this regime. If we want justice, we must retaliate against this regime. Continue your revolt and anger, don't accept anything else apart from independent, respectful governments."

The revolts of the Arab spring have posed a strategic problem for al-Qaida, which has always adamantly rejected democracy and nationalism as western inventions that weaken and divide the Islamic world.

In July, Zawahiri urged Syrian protesters to direct their movement also against Washington and Israel, denouncing the US as insincere in showing solidarity with them.

This month another video featuring Zawahiri appeared on Islamist forums, announcing that the Somali militant group al-Shabaab was joining its ranks, in an apparent attempt to boost morale and sharpen the threat to western targets.

Since its founding in 1988 al-Qaida has tried to appropriate local struggles and fuse them into one broad "global jihad".