UPDATE 3-Gunmen free more than 1,800 inmates in attack on Nigerian prison

* Prison break carried out by armed group
* Nigeria is battling multiple security challenges
* Police blame secessionist group
* Separatists deny involvement in attack

(Adds president's comment, background)
By Tife Owolabi
YENAGOA, Nigeria April 6 (Reuters) - More than 1,800
prisoners are on the run in southeast Nigeria after escaping
when heavily armed gunmen attacked their prison using explosives
and rocket-propelled grenades, the authorities said.
Nigerian police said it believed a banned separatist group,
the Indigenous People of Biafra (IPOB), was behind the attack in
the city of Owerri, but a spokesman for the group denied
involvement.
The secessionist movement in the southeast is one of several
serious security challenges facing President Muhammadu Buhari,
including a decade-long Islamist insurgency in the northeast, a
spate of school kidnappings in the northwest and piracy in the
Gulf of Guinea.
Buhari said the attack, in a city near the oil-rich Niger
Delta region that is the mainstay of Africa's top crude exporter
and biggest economy, was an "act of terrorism". He ordered
security forces to apprehend the fleeing prisoners.
The attackers stormed the facility at around 2:15 a.m. (0115
GMT) on Monday, according to the Nigerian Correctional Service
said.
"The Owerri Custodial Centre in Imo state has been attacked
by unknown gunmen and forcefully released a total of 1,844
inmates in custody," its spokesman said in a statement late on
Monday.
The police said attackers used explosives to blast the
administrative block of the prison and entered the prison yard.
"Preliminary investigations have revealed that the
attackers... are members of the proscribed Indigenous People of
Biafra (IPOB)," said Frank Mba, a spokesman for the Nigeria
Police Force.
IPOB wants independence for a region in southeast Nigeria it
calls Biafra. One million people died in a 1967-70 civil war
between the Nigerian government and secessionists there.
In recent months security in the region has deteriorated.
Several police stations have been attacked since January, with
large amounts of ammunition stolen and reports of the IPOB's
paramilitary wing, the Eastern Security Network (ESN), clashing
with the military.
But an IPOB spokesman told Reuters the group did not carry
out the prison raid.
"IPOB and ESN were not involved in the attack in Owerri, Imo
state. It is not our mandate to attack security personnel or
prison facilities," the IPOB spokesman said in a phone call.