TIMELINE: Messages attributed to Ayman al-Zawahri

(Reuters) - Al Qaeda's second-in-command Ayman al-Zawahri urged Muslims in an audio message posted on Tuesday to back the self-styled Islamic State in Iraq which he said still posed the biggest threat to U.S.-led forces.

At least 50 messages have been broadcast by Osama bin Laden, Zawahri and their allies since the September 11 attacks in 2001. The last message from bin Laden was last month.

Following is a chronology of major statements attributed to Zawahri in the last year.

May 23, 2007 - Zawahri eulogizes Mullah Dadullah, a Taliban leader killed in Afghanistan earlier in the month, and urges his followers to pursue his fight against U.S.-led forces.

June 25 - Zawahri urges all Muslims to support Hamas in its battles with Palestinian President Mahmoud Abbas and Israel.

July 4 - Zawahri, in a new video, calls for the overthrow of "corrupt" Muslim governments and talks about unity in jihad.

July 10 - Zawahri threatens more attacks on Britain two weeks after failed bombings in London and Glasgow.

July 11 - Zawahri, in an Internet recording, calls for revenge over the Pakistani government assault on an Islamabad mosque that killed more than 70 Islamists.

December 14 - Zawahri, in an audio recording, accuses Arab leaders of betraying the Palestinians by attending a Middle East peace conference in the United States and urges Muslims to keep fighting. Zawahri denounced the roles of Saudi Arabia and Egypt, saying they were serving Israel and U.S. interests.