Nigerian military says it arrested five suspects in an Islamist militant attack in a Catholic church that killed 40 people in early June, Chief of Defence Staff Leo Irabor said on Tuesday.

Federal authorities have said they suspected insurgent group Islamic State West Africa Province (ISWAP) carried out the massacre of members of a congregation inside the St Francis Catholic Church in Ondo state on June 5.

ISWAP is waging an insurgency in the Northeast but claims that it carried out the attack far away from its enclave have raised concern that the group is expanding its footprint in Nigeria.

Irabor said in a statement in Abuja that the attackers were arrested during joint operations involving the armed forces, the Department of Security Services and police. He did not say where and when the arrests were made.

He said he could not parade the suspects due to ongoing investigations.

"I will like to say that in due course, the world will see them and others who are behind other daring attacks in the country," Irabor said.

Rotimi Akeredolu, the governor for Ondo state, said a person who had provided accommodation to the suspects before the attack was also arrested.

ISWAP has claimed responsibility for a string of low level attacks as well as daring jail break in Abuja in early July that freed more than 400 inmates.

Reuters