* Extra fighters arrive overnight at al Qaeda stronghold
* Air forces strikes, militant ambushes kill at least 20
* Largest offensive against al Qaeda's AQAP since May 2011
* At least six al Qaeda-linked fighters, 14 soldiers killed
* More than 2,500 people flee homes to escape deadly clashes
By Mohammed Ghobari
SANAA, Jan 29 (Reuters) - Hundreds of al Qaeda-linked militants arrived in southern Yemen on Tuesday to reinforce Islamist fighters facing a major government offensive following the breakdown of talks to free three Western hostages, an official and residents said.
Air strikes against militant targets in the al Qaeda stronghold of al-Manaseh and ambushes by the Islamist fighters after Monday's army assault, killed at least six insurgents and 14 soldiers, including 11 killed by a suicide bomber..
More than 2,500 people had fled Manaseh, and were housed in schools in nearby villages and towns, the official said.
About 8,000 soldiers took part in the offensive on al Qaeda in the Arabian Peninsula's (AQAP) bastion in al-Bayda province, south of the capital Sanaa, a local official told Reuters.
Tackling lawlessness in the impoverished Arabian Peninsula state, which flanks the world's biggest oil exporter Saudi Arabia, is an international priority. The United States views Yemen as a front line in its struggle against al Qaeda.
The Islamist reinforcements were mostly from Abyan province, scene of heavy fighting in May 2012 that drove AQAP fighters from several southern towns, the official and local residents said.
"I saw dozens of bearded men, not locals, carrying guns," a resident who would only identify himself as Abbad said of the Islamist militants. "People say they are mujahideen (holy fighters) coming from Abyan."
A Finnish couple and an Austrian man, who were studying Arabic in Yemen, were snatched last month by tribesmen in the capital Sanaa. They were later sold to al Qaeda members, and taken to al-Bayda, a Yemeni official told Reuters this month.
The assault on Manaseh began a day after representatives of the 15 countries on the U.N. Security Council flew to Yemen in a show of support for a U.S.-backed power transfer deal in danger of faltering and plunging the country further into chaos.
Militants linked to AQAP, which U.S. officials believe is one of the most dangerous and active branches of the global network, were emboldened by widespread chaos in Yemen after an uprising in 2011 against former President Ali Abdullah Saleh.
Yemen's armed forces made their last major assault on AQAP in May 2011, a few months after Saleh stepped down under a Gulf-brokered power transfer deal.
There have been dozens of killings of security and military officials by suspected al Qaeda gunmen in the past year, suggesting AQAP remains resilient despite increased U.S. drone strikes and an onslaught by government forces.
Yemen has struggled to restore normality since President Abd-Rabbu Mansour Hadi was elected in February 2012 following a year of protests that forced his predecessor Ali Abdullah Saleh to step down after 33 years in power. (Editing by Louise Ireland and Sami Aboudi)