Al-Qaeda leader Ayman al-Zawahri has called on all Muslims to back the rebels in Syria, according to an audio recording posted on the internet yesterday.

He said the ousting of President Bashar al-Assad would bring them closer to the ultimate goal of defeating Israel.

Speaking on the anniversary of the 11 September attacks, Mr Zawahri criticised Muslim governments in the Middle East and in Asia for failing to pursue the cause of political Islam.

He chastised the new leadership in Egypt in particular for sticking to its 1979 peace treaty with Israel and Pakistan, which he described as a "government for sale and an army for rent".

The 2011 Arab Spring revolutions have redrawn the political landscape in the Middle East, bringing in Islamist governments in Tunisia and Egypt and increasing the influence of Islamist political groups throughout the region, which Western governments have watched with concern.

Mr Zawahri said the United States was propping up Assad because it feared the rise of another Islamist regime to threaten its ally Israel.

"Supporting jihad in Syria to establish a Muslim state is a basic step towards Jerusalem, and thus America is giving the secular Baathist regime one chance after another for fear that a government is established in Syria that would threaten Israel," he said.

More than 20,000 people have been killed in the 18-month uprising against Assad, who claims that his government is battling militants who want to set up an Islamist state.

The protest started as a pro-democracy protest movement but has since turned into an armed conflict with sectarian aspects.

Mr Zawahri, who took over as al-Qaeda chief after Osama bin Laden was killed last year, said "the Islamic nation" needed to focus on the goal of helping to "liberate Palestine" - a reference to Israel and the occupied territories where there are zones of Palestinian self-rule.

He said governments should annul peace treaties with Israel, criticised Turkey, Iran and Arab governments in the Gulf, and ridiculed Palestinian President Mahmoud Abbas for seeking peace with the Jewish state.

He singled out Egypt, saying the Muslim Brotherhood-led government was serving Israel by guarding its borders according to the terms of the Camp David peace treaty.

"I appeal to the honourable members of the Egyptian army, and there are many of them, not to be guards for the borders of Israel, and not to defend its borders or participate in besieging our people in Gaza," he said.

Egypt has launched a campaign against Islamist militants in Sinai after an attack that killed at least 16 soldiers last month.