According to reports, attackers infiltrated the congregation before shooting at people who had gathered for mass inside the church.
Attacks on places of worship are “abhorrent”, continued Mr. Guterres, who also urged the Nigerian authorities to spare no effort in bringing those responsible to justice.
To date, no group has claimed responsibility for the attack, which is considered unusual for southwest Nigeria.
It is a more peaceful region than the country’s north, where there have been numerous kidnappings for ransom by bandits, and past attacks on churches by separatists.
Echoing Mr. Guterres’s condemnation of the killings, the UN Alliance of Civilizations (UNAOC) called for the mutual respect of all religions and faiths, and for greater communal efforts to foster peace.
Bid to keep worshippers safe
The UN agency is behind a plan of action to keep places of worship safe, and to allow people of all faiths to practise their religions in peace.
In light of the attack, UNAOC High Representative Miguel Moratinos urged all governments to support the plan’s implementation.