ADEN, Yemen - More than 200 suspected al-Qaida gunmen have wrested control of the south Yemeni city of Zinjibar after heavy fighting with security forces that left 16 dead, an official said on Sunday.

The Yemeni opposition immediately accused embattled President Ali Abdullah Saleh of having allowed Zinjibar, capital of Abyan province, to fall to the gunmen to raise fears concerning al-Qaida and boost his flagging international support.

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The suspected al-Qaida fighters "were able to gain control of the city of Zinjibar ... and took over all government facilities", except for the headquarters of the 25th mechanized brigade, which is besieged by militants, the security official said.

Witnesses said that the gunmen were battling members of the brigade on Sunday.

"We will fight until the last bullet, and we will not surrender to the gunmen who killed our colleagues," an officer from the brigade said.

Residents said heavy fighting in the city on Friday and Saturday, and said the attackers had freed dozens of prisoners from the main jail in Zinjibar.

One witness said on condition of anonymity that the gunmen executed soldiers who surrendered, and that residents were not able to bury them.

The United States is concerned that tribal rivalries are complicating efforts to reach a power transfer deal in Yemen and believes al-Qaida is trying to exploit instability there, senior US officials said on Saturday.

The US remains in close contact with European and Gulf allies and continues to review options to increase pressure on Yemeni President Ali Abdullah Saleh to sign an agreement to step down, officials said.

The US government weighed in cautiously on Saturday even as Yemen's government and armed tribesmen demanding Saleh's ouster agreed to end clashes that had brought the Arabian Peninsula country to the brink of civil war. The truce did not resolve the country's wider political crisis.

Informal ceasefire

An informal ceasefire between Saleh's security forces and a tribal group brought a pause in fighting on Saturday.

Fighting this week has killed some 115 people, prompted thousands of residents to flee Sanaa and raised the specter of chaos that could benefit the Yemen-based branch of al-Qaida and threaten adjacent Saudi Arabia, the world's top oil exporter.

The latest violence, pitting Saleh loyalist forces against members of the powerful Hashed tribe led by Sadeq al-Ahmar, was the bloodiest since unrest erupted in January and was sparked by Saleh's refusal to sign a power transfer deal.