* Al Qaeda poses challenge to attempts to restore stability
* Witnesses report heavy fighting between army and militants
* Air strikes this month targeted "terrorist elements" (Updates with quotes from Yemeni president on AQAP)
By Mohammed Mukhashaf
ADEN, April 29 (Reuters) - Yemeni troops, backed by air force planes, launched a major offensive against al Qaeda strongholds in the south of the country on Tuesday to try to eradicate the Islamist militant group that had killed hundreds since 2011.
A local official said at least five Yemeni government soldiers were killed in the fighting, in which hundreds of volunteers from a local militia known as the Popular Committees are taking part.
The operation came less than two weeks after Yemen said some 55 suspected militants died in a series of air strikes on the militants' main hideouts in southern Yemen.
"Heroes of the armed and security forces, in cooperation with the Popular Committees, are determined to destroy the remnants of the terrorist elements in al-Mahfad area," Abyan governor Jamal al-Aqel was quoted on the Defence Ministry website as saying.
Militants from Al Qaeda in the Arabian Peninsula (AQAP) and its allies in the local Ansar al-Sharia group fled to the mountainous area in 2012 after Yemen's army, backed by the United States, drove them out of cities they had seized in 2011.
The insurgents have posed a major challenge to government efforts to restore stability to the U.S.-allied country since long-serving president Ali Abdullah Saleh was forced to step down in 2012 after months of pro-democracy protests.
The militants have carried out dozens of bombings, suicide attacks and commando-style raids against military installations, government facilities and foreign nationals, killing hundreds of people since 2012.
Local residents said on Monday the army had been massing troops near the town of al-Mahfad in preparation for a major offensive.
Local news websites have reported that several soldiers were captured by militants in the early stages of the fighting and photos were posted online of men in uniform sitting in the back of a truck, between masked men holding an al Qaeda banner.
It was not immediately possible to verify the reports.
The operation appears to have come as a follow up on a series of air raids on April 19 and 20, which Yemen said killed some 65 suspected militants, including 55 in southern regions and 10 in the central province of al-Bayda.
The Defence Ministry said those air strikes targeted "terrorist elements (who) were planning to target vital civilian and military installations".
On Tuesday, Yemeni president Abd Rabbu Mansour Hadi said 70 perecent of al Qaeda fighters were foreigners.
"Whoever is doubtful of that among our brothers, should go to the morgues in hospitals and see the corpses of people whose countries have refused to accept them. They're from Brazil, the Netherlands, Australia, France and various countries," Hadi told graduates at the police academy.
Local sources said unmanned drone aircraft had been seen above the target areas before the strikes earlier this month. The United States acknowledges using drone strikes to target AQAP in Yemen, but it does not comment on the practice.
Those air strikes came after an online video was posted on the Internet showing AQAP leader Nasser al-Wuhaishi addressing a large crowd of fighters in an undisclosed mountainous region of Yemen and vowing to attack the United States.
U.S. officials credit the drone strategy for the fact that AQAP is no longer able to control territory in Yemen as it did in 2011. But critics, including some Yemenis and U.S. politicians, say the strikes and civilian casualties are increasing sympathy for AQAP and resentment against Washington.
Saudi Arabia also watches AQAP with concern, since the branch was founded by citizens of both countries and has sworn to bring down its ruling al-Saud family.
(Reporting by Mohammed Mukhashaf; Writing by Sami Aboudi and Yara Bayoumy; Editing by Angus McDowall and Alison Williams)