By Felix Onuah
ABUJA, Aug 21 (Reuters) - Nigeria's President Muhammadu Buhari said on Monday he would stop separatist activists spreading unrest, seeking to reassert his hold on power after returning from his second stint of medical leave this year.
Buhari, looking thin but sounding stronger than his last broadcast in June, said he had been troubled by social media posts calling for splits in a country made up of a mostly Muslim north and a Christian south.
Since Buhari travelled to London for treatment of an unspecified condition on May 7, campaign groups have stepped up calls for a separate southeastern state known as Biafra, evoking memories of a conflict there that killed hundreds of thousands in the 1960s.
Militants and community groups have also called for the independence of the restive southern Niger Delta oil hub, saying it should receive a greater share of Nigeria's energy wealth.
Islamist Boko Haram militants are fighting for a separate caliphate in the northeast.
Buhari, a northern Muslim, has spent more time out of Nigeria so far in 2017 than he has inside the country, prompting fears that he is not well enough to run Africa's largest economy, which is mired in its first recession in 25 years.
The former military ruler's refusal to disclose details of his illness have prompted calls for greater transparency.
In a televised speech, his first since his return on Saturday, the 74-year-old said he followed events in Nigeria on a daily basis.
"I was distressed to notice that some of the comments, especially in the social media have crossed our national red lines by daring to question our collective existence as a nation," said Buhari.
"Nigeria's unity is settled and not negotiable. We shall not allow irresponsible elements to start trouble," he said, adding that some ethnic violence was "fuelled by political mischief makers".
Hours later, his spokesman said Buhari had written to lawmakers to notify them of his return to office.
"AGITATIONS AND THREATS"
Buhari first left for medical leave in Britain this year in January, staying for nearly two months. Both times he handed over power to his deputy, Christian southerner Yemi Osinbajo.
He also travelled to London to see a doctor in June last year over what his officials described as an ear infection.
Underlining the divisions simmering during his term in office, the Ijaw Youth Council, an organisation which represents the largest ethnic group in the Niger Delta, issued a statement on Monday contradicting the president, saying Nigeria's unity was "negotiable"
"We call on President Buhari to have a change of approach towards the resolution of the ongoing agitations and threats to national security by adopting constructive engagement to resolve the issues," it said in a statement.
Osinbajo held talks in June with regional leaders in an effort to quell tension after Muslim activists demanded the eviction of Igbo people from the north over their calls for the creation of a separate Biafra.
Nigeria has about 180 million inhabitants, split roughly equally between Christians and Muslims. About 250 different ethnic groups mostly live peacefully side-by-side.
The OPEC member has taken a hit from low oil prices and the impact of attacks on energy facilities last year in the Niger Delta.
Buhari, who won an election in March 2015, said it was in the interest of Nigerians to come together to face the challenge of economic security and promised to "reinforce and reinvigorate" the fight against Boko Haram insurgents in the northeast.
Suicide bomb attacks and raids by the Islamist militants have increased in frequency over the last few months, killing at least 170 people since June 1, according a Reuters tally. (Reporting by Felix Onuah, Paul Carsten and Alexis Akwagyiram in Lagos; Additional reporting by Tife Owolabi in Yenagoa; Editing by Robert Birsel and Andrew Heavens)