Italian police said Thursday they had arrested 15 African migrants after witnesses said they had thrown 12 passengers overboard following a brawl between Muslims and Christians on a boat heading to Italy.

The victims were "of Christian faith, compared to their attackers who were of Muslim faith," police in the Sicilian city of Palermo said in a statement, saying the 15 people were arrested on suspicion of "multiple aggravated murder motivated by religious hate".

The 219 schoolgirls who marched through Nigeria's capital on Tuesday said they will never tire of fighting for the rights of their kidnapped "sisters" who cannot speak for themselves.

Each girl carried a placard bearing the name of one of the hostages still missing after they were kidnapped by Boko Haram Islamist militants in the northeast town of Chibok one year ago.

Nigeria's president-elect Muhammadu Buhari cautioned Tuesday that he could not make promises on the return of 219 schoolgirls kidnapped by Boko Haram exactly a year ago.

"We do not know if the Chibok girls can be rescued. Their whereabouts remain unknown. As much as I wish to, I cannot promise that we can find them," he said in a statement.

Nigeria's president-elect Muhammadu Buhari on Tuesday cautioned he could not make promises on the return of 219 schoolgirls kidnapped by Boko Haram, as the country marked the first anniversary of their abduction.

The comments by Buhari, who takes office on May 29, stand in contrast to outgoing President Goodluck Jonathan, who has repeatedly said the girls will be found, and the military, which said last year it knew where the teenagers were being held.

Boko Haram has kidnapped at least 2,000 women and girls since the beginning of last year, Amnesty International said on Tuesday, a year on from the mass abduction of 219 Nigerian schoolgirls.

The kidnapping of the teenagers from Chibok, in the northeastern state of Borno, on April 14 last year brought unprecedented world attention to the brutality of the insurgency.

Nobel peace laureate Malala Yousafzai on Monday criticized Nigerian and world leaders for not doing enough to help free 219 schoolgirls kidnapped a year ago by Boko Haram militants.

"In my opinion, Nigerian leaders and the international community have not done enough to help you," she said in a letter to the teenagers, on the eve of the first anniversary of their abduction.

Nigeria's ruling party on Monday won back control of the key oil-producing state of Rivers, official results showed on Monday, after rigging claims by the main opposition.

Nyesom Wike, from President Goodluck Jonathan's Peoples Democratic Party, won 1,029,102 votes, while Dakuku Peterside, of the All Progressives Congress (APC), won 124,896, the electoral commission said.

The party of Nigeria's president-elect Muhammadu Buhari on Sunday won governorship elections in the economic capital Lagos, fighting off a tough challenge from outgoing President Goodluck Jonathan's party.

Akinwunmi Ambode of the All Progressives Congress took 811,994 votes in the key battleground state, beating out Jimi Agbaje of the Peoples Democratic Congress (PDP), who won 659,788 votes.

A typical day for Deborah includes classes on a manicured university campus and exercise in the evening -- basketball, volleyball or aerobics. On weekends, she studies, swims or just relaxes.

But the teenager's life now is one that was unimaginable 12 months ago.

Polling stations opened across Nigeria on Saturday for gubernatorial and state assembly elections, two weeks after a historic first opposition win in the presidential race.

Some 760 candidates are in the running for 29 governor and deputy governor positions, while 5,290 hopefuls are seeking local assembly seats in all 36 of Nigeria's states.