Nigeria has arrested suspects in an Islamist militant attack in a Catholic church that killed 40 people two months ago, Chief of Defence Staff General Leo Irabor told local media on Tuesday.

Irabor said the attackers were arrested during joint operations involving the armed forces, the Department of Security Services and police, privately owned Channels TV reported.

It was not immediately clear how many suspects had been taken into custody or where and when the arrests were made.

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