More than 20 casualties were caused Sunday in an attempted jailbreak by Boko Haram detainees in the Nigerian capital Abujia, the West African country's secret police disclosed.

The deaths and injuries were resulted from a shootout when the inmates were trying to escape from the prison.

Two security operatives sustained serious injuries in the incident, spokesperson of the Department of State Security (DSS) Marilyn Ogar said in statement.

"Following our earlier releases, it has been established that 18 casualties were recorded, while two Service personnel sustained serious injuries," she said.

Three other detainees sustained gunshot injuries and later died, Ogar added, noting that investigation was ongoing to ascertain the immediate and remote causes of the attempted jailbreak.

Security operatives in Nigeria foiled a jailbreak by suspected members of the Boko Haram sect at the secret police headquarters.

Ogar had said the incident occurred when the services' suspect handler went to the detention facility within the headquarters to feed suspects.

"One of the suspects attempted to disarm the handler by hitting him at the back of his head with his handcuff," she added.

Boko Haram, a sect which has launched many deadly attacks since 2009, has proved to be the major security threat in Nigeria, Africa's most populous country.