CAIRO -- The leader of al-Qaida has urged Muslims to kidnap Westerners to exchange for imprisoned jihadists, including a blind cleric serving a life sentence in the United States for a 1993 plot to blow up New York City landmarks.

In an undated two-hour videotape posted this week on militant forums, the Egyptian-born jihadist Ayman al-Zawahri also urged support for Syria's uprising and called for the implementation of Islamic Shariah law in Egypt.

He said that abducting nationals of "countries waging wars on Muslims" is the only way to free "our captives, and Sheik Omar Abdel-Rahman," the Egyptian cleric.

"This is the only language which they understand," said al-Zawahri, appearing in his customary white turban and robe. "We will keep on seizing more ... until we free our captives."

There was little clue to his whereabouts from the video, shot against a backdrop of brown curtains.

He periodically releases video and audio statements. Two weeks ago, an audio recording by him urged holy war over an amateur anti-Islam film produced in the United States. He released a video on this year's anniversary of the Sept. 11, 2001, attacks, claiming that his warriors "defeated America in Iraq".

Freeing Abdel-Rahman has become a rallying cause for Islamic militants and jihadists. A group named after him has claimed responsibility before for a June assault on the U.S. Consulate in Benghazi, which caused no casualties. It caused no casualties, but a bigger attack on Sept. 11 claimed lives of four Americans including the U.S. Ambassador in Libya Chris Stevens.

Relatives and supporters of Abdel-Rahman have been holding a sit-in next to the U.S. Embassy in Cairo for months. Egypt's new Islamist President Mohammed Morsi, under pressure from leading Egyptian jihadists recently released from prison, vowed to push for his release.

Al-Zawahri appeared to be following Egypt's debates over the country's political future, as he called upon ultraconservative clerics in Egypt to ensure clear mention of Islamic Shariah law in the new constitution. Members of Egypt's Salafi trend have been pushing the Muslim Brotherhood, the Islamist group from which Morsi hails, to make the role of Shariah explicit. Liberals fear that the Islamist groups will insert language that can be used to curb freedom of expression and the rights of women and minorities.

"Shariah must be the source of legislation ... This must be stated," Zawahri said. "This is the first step to cleanse the constitution and laws in order to implement Islamic Shariah law."

Egypt's old constitution and most drafts of the new one would include some reference to "Shariah" or the "principles" of Shariah, but the exact phrasing could have a major effect on future court rulings on the constitutionality of laws.

Al-Qaida leader also called on Muslims, especially in countries bordering Syria, to support the uprising there.

"I urge Muslims everywhere ... to rise up to support the brothers in Syria ... to get rid of the cancerous criminal regime," he said. "Syrian people have the right to defend themselves with all means."

The transformation of Syria's uprising into an open war has given an opportunity to foreign fighters and extremists to play a larger role in the uprising, analysts say. President Bashar Assad's regime has long blamed foreign "terrorists" for the country's crisis.

Al-Zawahri accused international community of indirectly approving the killing of Syrians.

"The international community ... is giving Assad a license to kill and one chance after the other to curb the Syrian revolution," he said. "They are afraid of a government that seeks victory for Islam and Muslims."

Syria's most Sunni rebels have received support from fellow Sunnis in the Gulf, while Assad's regime, dominated by a Shiite offshoot sect, is allied with Shiite-led Iran.

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