Three Italian nuns were found killed over the weekend in a parish in the north of Burundi's capital, officials and a priest in the African state said.

Police said two suspects had been detained for questioning.

Father Mario Pulicini, responsible for the parish in a northern suburb of Bujumbura, named two of the nuns as Lucia Pulici, who was 75 and due to celebrate her birthday on Monday, and Olga Raschietti, 82. He said they were found dead in their dormitory on Sunday.

The third nun, 79-year-old Bernadetta Boggian, was found early on Monday morning, he told Reuters, adding that the killings appeared to happen at two separate times.

"It is very difficult to know the reason behind the killing, but nothing can justify it," Father Mario said.

Italy's Foreign Ministry also reported the three deaths.

The Catholic diocese in Parma, Italy, said on its website that the death of Pulici and Raschietti appeared to have been "the tragic outcome of an armed robbery by a mentally unbalanced person".

Those two nuns had served in Burundi for seven years, after working several years in the east of another central African state, the Democratic Republic of Congo.