U.S. Defense Secretary Leon Panetta calls the Syrian military attack on the country's largest city, Aleppo, a nail in President Bashar al-Assad's coffin.

Panetta said during a flight to Tunisia Sunday that what President Assad has been doing to the Syrian people makes it clear his regime has lost all legitimacy and is coming to an end.

The U.S. defense chief flew to Tunis Sunday to begin a five-day trip to Tunisia, Egypt, Israel and Jordan.

Panetta said he commends the Tunisian people on the success of their revolution. He also commended the Tunisian army which, unlike the Syrian military, played what Panetta called a positive role in the transition.

Tunisians, demanding more human rights and better living conditions, forced long-time President Zine El Abidine Ben Ali to flee the country in January 2011. Those protests served as an inspiration for a wave of revolt that spread through the Arab world last year.