* Militants melted away after army retook southern towns
* Regroup into "sleeper cells" near towns, hide in hills
* New wave of deadly attacks challenge security moves
* Resourceful militants pose wider threat to Saudis, U.S.
A U.S.-backed military onslaught may have driven Islamist militants from towns in Yemen they seized last year, but many have regrouped into "sleeper cells" threatening anew the areas they vacated, security officials and analysts say.
The resilience of Al Qaeda in the Arabian Peninsula (AQAP), despite increased U.S. drone strikes to eliminate militants, is worrying for top oil exporter Saudi Arabia next door and the security of major shipping lanes in the seas off Yemen.
When a nationwide uprising against autocratic rule erupted last year, tying up security forces and causing a power vacuum, militants charged into the major south Yemen towns of Zinjibar, Jaar and Shuqra and set up Islamic "emirates".
To broad their appeal, the militants renamed themselves Ansar al-Sharia (Partisans of Islamic Law), appointed spokesmen to deal with the media and put up signposts and flags. Poverty, unemployment and alienation from a central government seen as aloof and corrupt spurred some young men to join the cause.
Residents said the militants included Saudis, Pakistanis, Egyptians, Chechens and Somalis, hinting at the international scope of the jihadi threat to Saudi and Western interests.
After President Ali Abdullah Saleh finally bowed to popular revolt and stepped down in February, the U.S.-backed Yemeni military swept in and wrested back southern towns from the militants, sometimes after heavy fighting.
But the south, where resentment of tribal domination from the north has long run high and a separatist movement revived in 2007, has since become a more dangerous place, residents say.
STEALTHY SHIFT IN TACTICS
"After their sudden withdrawal from Abyan, it ended with no one victorious or defeated," said Aden-based commentator Madyan al-Maqbas. "They had suddenly come, they took over, then they fled to the hills, and they left behind sleeper cells."
A rash of deadly violence in the major southern province of Abyan ensued, indicating that Ansar militants were still lurking in the vicinity of the towns they had once controlled.
Nine jihadis including the head of the Jaar "emirate" Nader al-Shaddadi were killed by a U.S. drone missile fired into a farmhouse where they were hiding just outside town on Oct. 19.
Five of the militants were teenagers from Jaar itself who had quietly moved into the farmhouse as a typical sleeper cell, a Yemeni security source told Reuters.
The next day, militants ambushed an army base in Shuqra, killing 16 soldiers, after apparently slipping out of lairs in the barren rugged mountains rearing up above the town.
"Most people are concerned about sleeper cells. We're aware of it and people have started to be more careful," said Hasan Ali Hasan, 35, from the Mansoura district of Aden where security forces raided some suspected "safe houses" this month.
In June, the commander of the army's southern division, a southerner who replaced a Saleh ally from north Yemen in March, was killed by a car bomb in a suburb of Aden, the sprawling main city and port in the south. Security forces subsequently uncovered numerous caches of suicide belts in the area.
There have been dozens of other attacks and kidnappings by undercover militants targeting security and military officials.
Yemeni security sources said the two leading figures in Ansar al-Sharia, Nader al-Shaddadi and Galal Bil-Eidy, are believed to be sheltering in mountains around Shuqra where they form the link between urban cells in Aden and AQAP commanders like Nasser al-Wuhayshi tucked away in mountains to the north.
They said such regional militant chieftains had activated sleeper cells to carry out assassinations of security officials in Aden and attacks like the one in Shuqra.
AL QAEDA IN THE ARABIAN PENINSULA
Formed in 2009, AQAP has carved out a reputation as al Qaeda's most formidable regional wing with suicide attacks on tourists, diplomats and operations against neighbouring Saudi Arabia, the world's top oil exporter, and U.S. targets abroad.
The sudden spread of al Qaeda in south Yemen was seen by many Yemenis as a ruse by Saleh, a northerner, to warn Western and regional backers of what would happen if their trusted strongman was no longer around to keep militancy in check.
Interim President Abd-Rabbu Mansour Hadi has begun replacing military and security chiefs in the south with southerners less connected to Saleh. The new command has been welcomed in Aden as more serious about challenging AQAP.
But the new commanders must still rely on tribal militias called "popular committees" inside southern towns and surrounding districts to stop militants returning in force.
Residents fear some militants could have infiltrated the committees, noting that the Ansar al-Sharia "emirate" in Jaar managed to negotiate a deal with the military that allowed many gunmen to leave unscathed.
Mohammed Sukain al-Jaadani, former head of a popular committee in Shuqra that helped organise tribes against the militants last year, is now trying to dissuade the region's youth from being tempted by jihadist ideology.
"After Jaar and Zinjibar, al Qaeda turned into sleeper cells. It's a danger for society, they are in many places. They threaten tribes and citizens," said Jaadani.
He has set up a Tribal Social Alliance in his home region in Hadramaut province bordering Saudi Arabia.
"We did good work with tribes, and we are still doing work to save our regions from al Qaeda and unknown people to reject destructive, terrorist ideas. They created a culture of violence and extremism. We're trying to help the authorities come back."
Jaadani recalled suspicious behaviour by Saleh's forces last year, including three incidents when fighter jets targeted his tribal force rather than militant positions. He said 30 were killed in that one attack, which the army said was a mistake.
"Al Qaeda was in all areas of Shuqra. But we cleaned them out and had a plan to bring a complete end to 'Ansar al-Sharia' here," Jaadani said. "But some officials in the state cut off supplies to us and we withdrew because of that.
"The former regime neglected its duty. When Saleh was still there, they didn't want tribes to succeed ... When we withdrew (last year), al Qaeda came back and took over Al-Kod and Shuqra, as if it was prearranged by the authorities."
Nasser al-Noba, a former army officer who helped relaunch the southern separatist movement, says militants have hunkered down in the Mahfad, Marakisha and Hatat mountains, inland from the flat coastal areas of Jaar, Zinjibar and Shuqra.
But they face an increasing risk of death from drones if they emerge, he said.
"They sometimes appear in the streets, they suddenly appear and disappear as if by remote control. They go around in landcruisers, with the stickers of al Qaeda on the doors. But since Saleh's fall, the drones have started to have an effect."
Drones have featured prominently in the Sanaa government's security strategy since Hadi took charge. Residents of towns once held by al Qaeda say many buildings were targeting during the campaign this year to evict the Islamists.
Some southern tribal figures have adopted the jihadi banner. One of them was U.S.-born Anwar al-Awlaki, who was killed along with his son by a U.S. drone in 2011, and Tarek al-Fadli, who was raised in Saudi Arabia and fought in Afghanistan.
Whatever the causes of al Qaeda's spread, and the brief public life of Ansar al-Sharia, analysts say the phenomenon of militancy would recede with a stronger, more efficient state presence and economic development creating jobs.
Despite AQAP's resourcefulness in south Yemen, Mohammed Qahtan of the Islamist Islah party in Sanaa is confident that al Qaeda, which killed 100 soldiers in a suicide attack on a Sanaa military parade in May, was now feeling more heat.
"Al Qaeda has been weakened since Hadi took over. The presence of the state in politics, development and security - filling the gaps the state had created - will tip the scales and change people's attitude towards al Qaeda."