* Momodu Bama was on most wanted list - army
* Boko Haram poses biggest security threat to Nigeria
* Insurgency weakened but unlikely to be stopped - experts (Changes throughout with defence statement, splits dateline)
By Lanre Ola and Imma Ande
MAIDUGURI/YOLA, Nigeria, Aug 14 (Reuters) - Nigerian soldiers have killed the second-in-command of the Islamist sect Boko Haram, an insurgency that has caused the deaths of thousands in the last three years, the defence ministry said on Wednesday.
It said Momodu Bama, who had a 25 million naira ($155,400) bounty on his head, was killed along with 17 other members of the sect during clashes with the military on Aug. 4 in Bama, a town in northeast Borno state.
"Momodu Bama has been personally leading the attacks against troops and innocent citizens in the communities of Yobe and Adamawa," the ministry said in a statement.
"A specialist in manning the anti-aircraft guns of the group, he is known to be vicious and heartless with a penchant for personally slaughtering and executing his victims."
The ministry said Bama was identified by other arrested militants.
Boko Haram's leader Abubakar Shekau, who is the only visible member of the group through appearing on Internet videos, says the army lies about victories over his fighters.
The military has announced the killing of senior members of the sect before, notably a spokesman called Abu Qaqa, only for a person using the same name to say he had not been killed.
Boko Haram, which wants to impose sharia law in Nigeria's north, as well as other spin-off Islamist groups, have become the biggest threat to stability in Africa's top oil exporter.
The group's name roughly translates as "Western education is forbidden."
The military in Adamawa state, to the south of Borno, said on Wednesday it had killed two other top Boko Haram commanders during a four-hour gun battle last week.
The two men were arrested near the town of Mubi last week and confessed they planned an operation in Taraba state to the south, Mubi Army Chief Beyidi Marcus Martins told reporters.
The men died in shooting that erupted after they took army officers to show them their hideout in Mubi. Several other sect members were also killed, Martins said.
Human rights groups have accused Nigerian soldiers of carrying out extra-judicial killings during their fight against Boko Haram. The army has always denied the accusations.
In mid-May, President Goodluck Jonathan declared a state of emergency and launched an offensive against the group in its stronghold in Borno, Yobe and Adamawa states in the northeast.
The insurgency was initially weakened but remains active.
Gunmen suspected of belonging to Boko Haram killed at least 44 people and wounded 36 after an attack during early Sunday prayers at a mosque in northeast Borno state.
Boko Haram's main targets are security forces or government officials but it has carried out several attacks on Christian and Muslim worshippers, as well as schools and markets.
"Despite the numerous arrests of suspected terrorists and continuous reports of the success of military offensives, there are still legitimate concerns over whether the ongoing security operations could really result in the resolution of the insurgency," a security source said on Wednesday. ($1 = 160.6 Nigerian naira) (Additional reporting by Camillus Eboh in Abuja; Writing by Joe Brock; Editing by Jon Boyle and Anthony Barker)