Twelve al-Qaeda militants have broken out of a prison in the southern Yemeni city of Aden, officials have said.

The militants, and two others, escaped through a six metre tunnel dug from the yard at the city's central prison.

The escapees were in the process of being tried for bank robbery or were charged with assassinations of security officers, the Yemeni officials said.

In June, al-Qaeda fighters raided the central jail in the southern city Mukalla, freeing dozens of prisoners.

Yemen's army has been fighting heavy gun battles with al-Qaeda militants in different parts of Yemen.

The country is experiencing unrest and political crisis on several fronts - insurgency in the north, a separatist movement in the south, nationwide protests calling for reform and free elections, and gun battles between different factions in the capital Sanaa.

President Ali Abdullah Saleh, who has governed for 32 years, agreed last month to hand his powers to his deputy and quit within 90 days.

The most intense fighting between the army and al-Qaeda is in the city of Zinjibar, the capital of Abyan province, where militants control much of the city.

The United Nations says that suspected al-Qaeda fighters in southern Yemen have displaced 45,000 people.