* Boko Haram is biggest threat to security in Nigeria
* Sect killed 20 in raids in northeast stronghold
* President sending more troops to fight insurgents (Adds details, quote, background)
By Lanre Ola
MAIDUGURI, Nigeria, Sept 7 (Reuters) - Nigerian soldiers have tracked down and killed 50 members of the Islamist sect Boko Haram in its northeastern stronghold, an army spokesman said on Saturday.
Army units mounted the operation after suspected Boko Haram fighters killed 20 people in two attacks on Wednesday and Thursday in villages in northeastern Borno state.
"Troops pursued the terrorists to their camps and with air support about 50 terrorists were killed in a shoot-out," army spokesman Sagir Musa told reporters in Maiduguri.
Boko Haram, which wants to impose sharia law in northern Nigeria, and other splinter Islamist groups, are the biggest threat to stability in Nigeria, Africa's top oil exporter.
The military sometimes exaggerates its successes and plays down its own casualties and the deaths of civilians, residents of Borno state and rights groups have said. Musa said the number of civilian casualties caused by the latest army offensive was not yet known.
More than 160 people were killed in violence linked to Boko Haram last month - one of the bloodiest periods since President Goodluck Jonathan declared a state of emergency and a military crackdown in three northeastern states in mid-May.
(Writing by Joe Brock; Editing by Angus MacSwan)