Nigeria's army has stepped up the hunt for hundreds of schoolgirls, abducted last month by  Boko Haram, in an attack condemned globally including by US first lady Michelle Obama.

The country's Defence Headquarters said on Saturday that two divisions of the military were now stationed in the border regions close to Chad, Cameroon and Niger to work with other security agencies.
At least 10 army search teams were trying to track down the girls in the remote far northeast, border guards were on high alert and the air force had so far flown at least 250 sorties.
Teams from the US, UK and China had also arrived in Nigeria to assist with the search. They included specialist teams in areas that include intelligence gathering, satellite imagery and hostage negotiations.
At least 200 schoolgirls remain missing and the government's slow response to the abduction has led to protests around the country.
Denial of warnings
Defence spokesman Chris Olukolade on Saturday also denied separate reports on Friday by Al Jazeera and Amnesty International that the Nigerian military had received advance warnings of the attack on the Chibok school, describing the reports as "unfounded".
Amnesty said on Friday it was told that security forces were given four hours' notice of the attack on April 14, but failed to reinforce the town.
Two politicians from Borno state, which borders Chibok, separately told Al Jazeera's Yvonne Ndege that the army had been given at least two hours' notice.
Makmid Kamara, a Nigeria researcher for Amnesty, on Friday said: "We received information and we spoke to a senior Nigerian military officer ... that they had received intelligence reports, even before local authorities and politicians got the information, that gunmen were on their way to the Chibok town."
Kamara told Al Jazeera that senior officials in Maiduguri and Dambua towns were alerted at about 7pm on April 14, and that information was given to senior military officers based in Dambua and Maiduguri.
"Later on, at 10pm on the same night of the 14th of April, local authorities, who Amnesty had spoken to, informed us, that they informed the local military command in Chibok town about the planned attack," Kamara said.
"When I spoke to one of the senior military officials, they informed me that they [had] informed their superiors, and requested for reinforcement. But the reinforcement did not come."
The school was attacked at 11.45pm.
"Only 17 troops were there to face the attack and they were outgunned and outnumbered," Kamara said.
"They had to flee for their lives together with some other villagers."
Ndege, reporting from Abuja, said the Nigerian people would be "extremely shocked and extremely disappointed" to think the Nigerian military knew in advance an attack was going to take place and most would find it "inexcusable".
"The question is, why would the military deliberately choose to ignore this SOS?" our correspondent said.
Michelle Obama, the wife of the US president, used the president's weekly address in the US to talk about the abductions, which she described as an "unconscionable act was committed by a terrorist group".
Mrs Obama was recently featured on Twitter holding a card which read "bring back our girls".
The Obama administration had in the past opposed imposing sanctions on Boko Haram, and refused to put the armed group in the United States' foreign terrorist organisation list.  
Carl Levan, an African security specialist at American University, however said that the "robust bilateral relationship" between the US and Nigeria prompted the Obama adminitration to help President Goodluck Jonathan find the abducted schoolgirls.    
Also on Saturday, Nigerian celebrities joined about 100 protesters in a march to the state governor's residence in Lagos to call for the rescue of the girls.