A car bomb outside the gate of a presidential compound in a southern Yemeni city has killed at least 25 people, a health official said, hours after the country's new president was formally inaugurated and vowed to fight al Qaida.

A security official said it was a suicide blast, and that it bore the hallmarks of an operation by the militant group. He did not provide a death toll. Both al Qaida and southern separatists are active in the region.

The blast came as Abed Rabbo Mansour Hadi was sworn in as president to replace long-time leader Ali Abdullah Saleh, following an election aimed at ending more than a year of political turmoil.

In his televised speech before parliament, Mr Hadi swore to keep up Yemen's fight against al Qaida-linked militants, who took advantage of the country's upheaval to seize control of several parts of the country.

He also pledged to work to bring home the thousands of internal refugees created by fighting between government troops, southern separatists, mutinous military units, tribal movements, and numerous other factions.

"One of the most prominent tasks is the continuation of war against al Qaida as a religious and national duty, and to bring back displaced people to their villages and towns," Mr Hadi said.

Hours later, the bomb exploded in the city of Mukalla in the province of Hadramout, part of formerly independent south Yemen, that federated with the north in 1990.

Ahmed al-Rammah, who witnessed the blast, said by phone from Mukalla that he saw a pick-up truck moving slowly to the gate as soldiers were coming out. Then it exploded, he said. The blast was followed by heavy gunfire from the surviving guards.

Newly inaugurated President Hadi has an onerous task ahead of him bringing stability to Yemen.

He must restructure powerful security forces packed with Saleh loyalists, launch a national dialogue that would include southern secessionists, and appease a restless religious minority in the north as well as disparate opposition groups in the heartland.