* Security source says 11 dead after prison attack
* 29 inmates escape in chaos - interior ministry
* Yemen grappling with growing al Qaeda threat (Adds number of inmates escaped)
By Khaled Abdullah
SANAA, Feb 14 (Reuters) - Eleven people were killed when attackers mounted a bomb, grenade and gun assault on the main prison in Yemen's capital on Thursday to free inmates, security sources and witnesses said.
Explosions and gunfire could be heard several kilometres away from the prison in northern Sanaa, which has al Qaeda members among its inmates. The biggest explosion rattled windows in the area.
"A terrorist group attacked the central prison," an Interior Ministry official said, according to comments published by the state news agency, adding there had been a car bomb followed by a gun attack on the facility.
"Guards managed to confront the terrorists and forced them to flee," the report said.
Eleven people were killed, a security source said. The Interior Ministry official said seven guards were killed and four wounded, while 29 inmates, including 19 jailed for terrorism-related crimes, escaped in the chaos.
The ministry released the names of the escapees and asked citizens to report any information to help capture them.
No one claimed responsibility for the attack, but Yemen is grappling with a growing threat from one of al Qaeda's most active wings, which has killed hundreds of people in assaults on state and military facilities in the past two years.
Some Yemeni news websites said al Qaeda was behind the attack.
Police sealed off the road to the airport, which runs through the neighbourhood where the prison is located, and thick smoke was seen rising above the area.
Earlier on Thursday, a British teacher was reported missing in Sanaa in what a Yemeni security source suggested could have been a kidnapping. The abduction of foreigners in Yemen is common.
The U.S. ally, with a population of 25 million, is trying to end nearly three years of political unrest, which began when mass protests erupted in 2011 against Ali Abdullah Saleh, the president of 33 years, who stepped down.
Interim President Abd-Rabbu Mansour Hadi has been facing other challenges in trying to restore stability to Yemen, which shares a long and porous border with top world oil exporter Saudi Arabia.
Yemen is also trying to deal with demands by southern separatists for independence and to quell rebels from the Shi'ite Muslim Houthi movement, which has been on an offensive to extend its control over the north. (Additional reporting by Mohammed Mukhashaf in Aden, Writing by Sylvia Westall; Editing by Gareth Jones and Robin Pomeroy)