Media playback is unsupported on your device Media caption President Goodluck Jonathan: "Wherever these girls are, we will surely get them out" 
 
 President Goodluck Jonathan has admitted that Nigerian security forces still do not know where more than 200 abducted girls are being held. 
 
 They were taken three weeks ago from their school in Borno state by suspected Islamist militants. 
 
 President Jonathan was speaking for the first time since their disappearance amid growing criticism of the response. 
 
 He has come under fire for not speaking earlier and his government has faced increasing anger from the public. 
 
 "We promise that anywhere the girls are, we will surely get them out," he said in a live TV broadcast. 
 
 The president said that despite searches by the army and the air force, the girls had not been found. 
 
 He asked for the co-operation of parents and the local communities in the rescue efforts, saying the "government needs assistance." 
 
 "It is a trying time for this country... it is painful," he added. 
 
 Image copyright AP Image caption There have been protests across Nigeria calling on the government to do more to free the girls 
 
 The BBC's Will Ross in Abuja says it appears somewhat astonishing that the girls cannot be found when there are reports they have been moved around in convoys of vehicles. 
 
 This seems to be a sign that there are parts of north-east Nigeria that are more or less off limits to the Nigerian armed forces, our correspondent says. 
 
 Islamist militants known as Boko Haram, whose name means "Western education is a sin", are believed to be behind the kidnapping of the girls from their school in Chibok. 
 
 They have waged a violent campaign in the north-east that has killed hundreds of Christians and Muslims. 
 
 President Jonathan dismissed the suggestion that negotiations were taking place to secure the release of the girls, saying it was impossible to talk to Boko Haram. 
 
 "You don't negotiate with somebody you don't know. The issue of negotiation has not come up," he said. 
 
 Image copyright AP Image caption Security has been tightened in Abuja after two recent attacks killed nearly 100 people in the capital 
 
 He said his government has spoken to the United States and several other world powers, including France, Britain and China, for help with its security issues. 
 
 "We are talking to countries we think can help us out. The United States is number one. I have talked to President Obama at least twice," he said. ||||| Image copyright AFP Image caption The kidnapping of the schoolgirls from Chibok has outraged the international community 
 
 Nigeria's President Goodluck Jonathan has called off a visit the town where more than 200 schoolgirls were abducted, officials say. 
 
 Sources had told the BBC he would stop in Chibok, in the north-east, on his way to a conference in France on the threat from Boko Haram militants. 
 
 But the visit was called off for security reasons, the officials said. 
 
 The president - under pressure over his government's failure to rescue the girls - will fly direct to Paris. 
 
 The BBC's Gabriel Gatehouse in Nigeria says the cancellation of this visit underlines just how fragile the security situation is in the north-east. 
 
 On Thursday, relatives of the girls called for their unconditional release by Boko Haram. 
 
 Mr Jonathan is said to have ruled out negotiations over a possible release of prisoners. 
 
 Nothing was seen of the girls for almost a month after they were taken from Chibok. 
 
 But on Monday the group released a video showing more than 100 of them and offering an exchange for prisoners. 
 
 UK Africa Minister Mark Simmonds said Mr Jonathan had "made it very clear that there will be no negotiation" at a meeting on Wednesday. 
 
 Image copyright AP Image caption A video emerged on Monday showing about 130 of the girls wearing hijabs and reciting Koranic verses 
 
 President Jonathan has been criticised for not visiting the town - more than a month after the girls were seized. 
 
 The president will travel to Paris to take part in a summit convened by French President Francois Hollande to discuss Boko Haram. 
 
 The leaders of Nigeria's neighbours - Benin, Cameroon, Niger and Chad - are scheduled to attend the summit on Saturday, which will also include representatives from the UK, US and EU. 
 
 A statement said delegates at the meeting will "discuss fresh strategies for dealing with the security threat posed by Boko Haram and other terrorist groups in west and Central Africa". 
 
 'Troubling atrocities' 
 
 Meanwhile, US officials on Thursday criticised the speed of Nigeria's response to the threat from Boko Haram. 
 
 Alice Friend, director for African affairs at the US defence department, said its security forces had been "slow to adapt with new strategies and new tactics". 
 
 Media playback is unsupported on your device Media caption John Simpson assesses the threat of Boko Haram 
 
 She also said the US was unable to offer aid to Nigeria's military because of "troubling" atrocities perpetrated by some units during operations against Boko Haram. 
 
 "We cannot ignore that Nigeria can be an extremely challenging partner to work with," Ms Friend said. 
 
 State of emergency 
 
 US drones and surveillance aircraft have been deployed to assist in the search for the schoolgirls, while the UK has sent a military team to the capital, Abuja, to work alongside US, French and Israeli experts. 
 
 The lower house of Nigeria's parliament, the House of Representatives, approved an extension of the state of emergency in the north-east states of Yobe, Borno and Adamawa on Thursday. 
 
 Image copyright AFP Image caption A US official said its cooperation with Nigeria's army was limited by its links to "troubing" atrocities 
 
 President Jonathan had requested a six-month extension, calling the security situation in the region "daunting" and saying he was concerned by the mounting loss of life among civilians. 
 
 The state of emergency, which still needs to be approved by the Senate, gives the military widespread powers such as detaining suspects, imposing curfews and setting up roadblocks. 
 
 On Thursday, there were reports of fresh attacks by suspected Boko Haram militants in Borno state. 
 
 A witness told the BBC's Hausa Service that there had been explosions in Gamboru Ngala, were some 300 people were killed last week in a massacre blamed on Boko Haram. ||||| ABUJA Nigeria's hunt for more than 200 abducted schoolgirls is not all that it seems. In public, an international operation is gathering pace while behind the scenes, officials say it is unlikely to deliver the success that global opinion demands. 
 
 The United States and Britain are helping Nigerian forces in the effort to liberate the girls taken from their school in Borno state a month ago by Boko Haram Islamist militants. 
 
 The pressure for results is huge, with the likes of Michelle Obama and film star Angelina Jolie supporting a social media campaign operating under the Twitter tag #BringBackOurGirls. 
 
 Washington has sent surveillance aircraft as well as assigning medical, intelligence, counter-terrorism and communications advisers to the mission. 
 
 But officials have little idea where the girls are, and acknowledge that if they are found, any rescue attempt would be fraught with problems. On top of that, morale is shaky among some of the Nigerian troops involved in the hunt who already have experience of Boko Haram as a formidable foe. 
 
 "We commend the effort by the #BringBackOurGirls protesters but it doesn't fit with the reality of the security situation we are facing," said a senior Nigerian military source who spoke on condition of anonymity. 
 
 Foreign experts are also pessimistic that the girls can be extricated from the rebels' clutches and returned to their homes in Nigeria's remote northeast where Boko Haram operates. 
 
 "I think a rescue is currently unlikely and unfeasible," said Jacob Zenn, a Boko Haram expert at U.S. counter-terrorism institution CTC Sentinel. 
 
 Until Monday, nothing had been seen of the girls since they were snatched from the village of Chibok near Nigeria's borders with Cameroon, Chad and Niger. 
 
 Then Boko Haram released a video showing more than 100 girls together in a rural location. In it, rebel leader Abubakar Shekau offered to exchange them for captured militants. 
 
 The video raised hopes that their location could be found using ground forces, state-of-the-art intelligence and surveillance planes. 
 
 Then an operation could be staged, perhaps with forces swooping from the sky like a British raid in Sierra Leone in 2000 to free soldiers held by militiamen, or Israeli commandoes' rescue of passengers from a jet hijacked to Entebbe, Uganda, in 1976. 
 
 However, such a scenario is unlikely this time. One source with knowledge of the search said the footage was probably taken at least 10 days ago, if Boko Harma's past videos are any guide. By now, the girls could be somewhere else as a group, or dispersed to many places. 
 
 VIRTUALLY UNDETECTABLE 
 
 The Sambisa forest, Boko Haram's stronghold, is a first target but it is not conducive to aerial search because it covers 60,000 square km (23,000 square miles), more than twice the size of Rwanda. The rebels know this area intimately and could spread the girls among local families, making them virtually undetectable by conventional security forces. 
 
 Two U.S. national security sources said initially the girls were separated into around three large groups but were subsequently scattered in smaller groups. Other experts said they could be in mountains near Gwoza on the Cameroon border. 
 
 If this is the case, some girls might be found before others, posing a dilemma for would-be rescuers. 
 
 "In the past, Boko have threatened, and maybe actually gone ahead with, killing hostages upon sensing the merest hint of possible rescue operations," said a security source. Rescuing some girls could add to danger faced by others still captive. 
 
 Another problem is time. Britain's minister responsible for African affairs, Mark Simmonds, said on Wednesday that it was "early days" in the rescue operation, yet the abduction happened on April 14 so rebels have had ample time to prepare for an international response. 
 
 A senior U.S. Defense Department official criticized Nigeria on Thursday for being too slow to adapt to the threat of Boko Haram. 
 
 TIRED OF FIGHTING 
 
 An incident this week highlighted the problems of the Nigerian army, whose 1st and 7th divisions have been deployed to Maiduguri, the capital of Borno state, to confront the rebels. 
 
 On Tuesday, gunmen ambushed a unit returning to Maiduguri from Chibok, killing at least four soldiers. Rebels were also killed. The next day soldiers fired into the air at Maiduguri barracks to protest about what they said was poor leadership. 
 
 Defense headquarters quickly stated that the situation had been resolved, but soldiers told Reuters they remained angry. 
 
 "These Boko Haram have special forces. They know how to shoot and maintain their grounds more than us. What is making it worse is our superior officers who are insensitive to the plight of the troops," said a soldier, who declined to be identified. 
 
 "We are tired of this whole thing because we are being killed every day and we don't get the required support and care from our superiors," another soldier said. 
 
 President Goodluck Jonathan will visit Chibok on Friday, senior government officials said. 
 
 Even coordinating an international effort faces difficulties in Nigeria, which recently overtook South Africa as the continent's biggest economy. Nigeria has close ties with Western powers but has historically resisted foreign military involvement on its soil. 
 
 One possible sign of differing approaches is that Simmonds, rather than the president himself, announced that Jonathan had ruled out any prisoner exchange for the girls' release. Nigerian officials have since declined to comment. 
 
 Ultimately, the girls' best hope may lie in dialogue but the road to talks remains uncertain because the rebels do not form a unified group. 
 
 Boko Haram is faceless and even Shekau heads just one of several loosely coordinated groups with differing objectives, said a senior official with knowledge of the northeast. 
 
 A Nigerian presidential committee set up last year for talks with the rebels dealt last year with Boko Haram proxies. But they were later denounced by other Boko Haram militants as impostors, according to Minister of Special Duties Tanimu Turaki, who leads the committee. 
 
 One security source in Abuja cautioned against raising false hopes. "It is time we removed the thought of a very happy ending to this situation," the source said. 
 
 (Additional reporting by Felix Onuah in Abuja, Isaac Abrak in Maiduguri and Mark Hosenball in Washington; Editing by David Stamp)
– The Wall Street Journal has a report raising hopes about the 200 or so Nigerian girls still missing after being kidnapped by Boko Haram. US surveillance planes have on at least two occasions spotted sizable groups of girls in remote locales in northeastern Nigeria. The girls had been moved by the time subsequent flights flew overhead, but the development suggests that instead of selling the girls into sex slavery as feared, Boko Haram is keeping them in the hopes of a prisoner swap with the government. In fact, the Journal quotes a Nigerian security adviser who says that Boko Haram leader Abubakar Shekau has given orders that "anybody found touching any of the girls should be killed immediately." President Goodluck Jonathan has publicly ruled out a prisoner exchange, but he is up for re-election in February and under heavy pressure to bring the girls home. Meanwhile, a human rights group has launched a campaign to provide educations for the girls who managed to escape, reports the Catholic News Agency.