Al-Qaida leader Ayman al-Zawahiri has called on Muslims to kidnap Westerners, join Syria's rebellion and to ensure Egypt implements sharia, SITE Monitoring reported on Saturday, citing a two-part film posted on Islamist websites.

The Egypt-born cleric, who became al-Qaida leader last year after the death of Osama bin Laden, spoke in a message that lasted more than two hours.

"We are seeking, by the help of Allah, to capture others and to incite Muslims to capture the citizens of the countries that are fighting Muslims in order to release our captives," he said, praising the kidnapping of Warren Weinstein, a 71-year-old American aid worker in Pakistan last year.

Zawahiri's message was first released on Wednesday, SITE said, just two weeks after the cleric issued a filmed statement calling for more protests against the United States over a California-made film mocking the Prophet Mohammad.

In his new message, he called on Muslims to ensure Egypt's revolution continued until sharia law was implemented and urged fellow Muslims to join the uprising against President Bashar al-Assad in Syria.

The release of his message had been delayed, he said, because of the "conditions of the fierce war" in Afghanistan and Pakistan where he said U.S.-led forces had intensified a bombing campaign.

President Barack Obama, whom Zawahiri described as a "liar" and "one of the biggest supporters of Israel," has stepped up the use of unmanned drones to target militants in both countries as well as in Yemen.

In a further attack on Western governments and international institutions, Zawahiri accused world powers of giving Syrian President Assad "a licence to kill" his opponents.

"The U.N., [former international negotiator on Syria] Kofi Annan and the Arab League give the al-Assad regime one opportunity after another to end the rising of jihadi, popular resistance against his oppression, injustice, corruption and spoiling," SITE reported Zawahri as saying.

Syria's anti-government rebels include Islamist groups that draw on foreign fighters.

"I incite Muslims everywhere, especially in the countries that are contiguous to Syria, to rise up to support their brothers in Syria with all what they can and not to spare anything that they can offer," he said.

Zawahiri, who led the Egyptian Islamic Jihad movement before joining al-Qaida, called on President Mohammed Morsi, the country's new Islamist leader, to explain his policies on Israel, Egyptian Christians and sharia law.

Islamist militants want Egypt to introduce sharia and to tear up a 1979 peace treaty with Israel and were dismayed when Morsi said he would appoint a Coptic Christian vice president.

"The battle in Egypt is very clear. It is a battle between the secular minority that is allied with the church and that is leaning on the support of the army, who are made up by [former President Hosni] Mubarak and the Americans ... and the Muslim ummah (nation) in Egypt that is seeking to implement sharia," he said.