UNITED NATIONS -- UN Secretary-General Antonio Guterres on Sunday condemned the terrorist attacks on worshipers in three churches in Surabaya, Indonesia.

The UN chief was appalled at reports that children were used to participate in the attacks, said Guterres' spokesman Stephane Dujarric in a statement.

Guterres expressed his condolences to the families of the victims and wished a swift recovery to those injured.

He reiterated the support of the United Nations to the government and people of Indonesia in their efforts to fight and prevent terrorism and violent extremism, including through the promotion of pluralism, moderation and tolerance, said the statement.

Suicide bombers who carried out deadly attacks on three churches in Indonesia's second-largest city on Sunday were a family of six that included two young children, police said, as the world's most populous Muslim nation recoiled in horror at one of the worst attacks on its Christian minority.

At least 13 people, including all six family members, died in the attacks in Surabaya and at least 41 were injured, according to police, in acts that Indonesia's president condemned as "barbaric".

The bombings were the worst to target churches in Indonesia since a series of attacks on Christmas Eve in 2000 killed 15 people and wounded nearly 100. Religious minorities in Indonesia, especially Christians, have been repeatedly targeted by militants.