Nigeria's President Goodluck Jonathan has ordered the closure of the country's land and sea borders before this weekend's general election, the interior ministry said on Wednesday.

The closure will take effect from midnight (2300 GMT Wednesday) until the same time on Saturday, after polling closes for presidential and parliamentary elections.

Boko Haram appears to have been weakened by a sustained regional fight-back but there are growing fears the group could target vulnerable people displaced by the violence, as elections approach.

More than 13,000 people have been killed in the bloody six-year insurgency, with some 1.5 million more forced to flee their homes within Nigeria and abroad.

U.S. President Barack Obama on Monday called on Nigerians to reject violence in this weekend's general election, describing the vote as a "historic opportunity" for progress in Africa's most populous country.

"I call on all Nigerians to peacefully express your views and to reject the voices of those who call for violence," Obama said in a video message addressed "to the Nigerian people."

Africa's most populous nation, Nigeria, goes to the polls to elect a new president next week after a controversial postponement, with the six-week delay seeing major gains against the Boko Haram jihadists.

Fourteen candidates, including the first woman, are eying the top job but the election is a two-horse race between incumbent Goodluck Jonathan and former military ruler Muhammadu Buhari.

Violence by Islamic extremists has led to staggering levels of misery and hardship in northeastern Nigeria, ranging from food shortages to families torn apart by relentless attacks, a top UN official said.

"We're seeing tremendous suffering," UN Assistant Secretary General Robert Piper told AFP in an interview this week.

Gunmen kidnapped the younger brother of Nigeria's oil minister and current OPEC president Diezani Alison-Madueke in southern Bayelsa state on Thursday, a police spokesman said.

The gunmen kidnapped Joseph Agama at around 1100 GMT in the Yenaka community of Yenagoa, which is the capital of the oil-rich state, said Asinim Butswat.

Boko Haram militants killed 11 people in an attack on Gamboru in northeast Nigeria as hundreds of Islamists flooded the town following the withdrawal of Chadian troops, witnesses said Thursday.

Three residents of Fotokol, a town in Cameroon but effectively attached to Gamboru, said the rebel onslaught began on Wednesday and there were indications that Boko Haram intended to occupy Gamboru again. 

Dozens of Nigerian women who were forced to marry Boko Haram fighters were reportedly slaughtered by their "husbands" before a battle with troops in the northeast town of Bama, multiple witnesses said Thursday.

Five witnesses who recounted the massacres to AFP said the Islamist militants feared they would be killed by advancing soldiers or separated from their wives when they fled the town.

The U.N. envoy for global education called Wednesday for a multimillion-dollar fund to provide education for children in emergencies and urged donors to start with $163 million to educate half a million Syrian children who are refugees in Lebanon.

Gordon Brown told a news conference Wednesday that it's time for decisive action to prevent millions of children from falling through the cracks and losing out on an education.

Amnesty International on Thursday said two major oil companies operating in Nigeria, Shell and ENI, reported a combined 553 oil spills in 2014, a "seriously alarming" figure that qualifies as a "national emergency."

The rights watchdog said that analysis of the latest figures released by both companies established that Anglo-Dutch Shell reported 204 spills last year, while the Italian major ENI admitted 349.