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"The window of negotiation is still open," said Minister of Special Duties Tanimu Turaki, head of an amnesty committee set up by President Goodluck Jonathan last year and charged with talking to the Boko Haram militants behind a five-year-old insurgency.
"The government had set up a committee to negotiate with Boko Haram, so if they have any negotiation to make it should be channelled through the committee," Turaki told Reuters by telephone.
Turaki declined to comment on possible talks over the kidnapping itself. Senior officials say the government is exploring options and there has been no commitment to negotiations for the release of the girls.
Boko Haram has killed thousands of people since 2009 and destabilised parts of northeast Nigeria, the country with Africa's largest population and biggest economy.
The abductions have triggered a worldwide social media campaign under the Twitter hashtag #BringBackOurGirls, and prompted the United States, Britain, France and Israel to offer help or send experts to Nigeria. U.S. surveillance aircraft were flying over remote areas of the northeast on Tuesday.
The amnesty committee's initial six-month mandate expired without holding direct talks with the rebels, though it has spoken to them through proxies, according to senior government officials. It has since been replaced by a standing committee empowered to conduct talks.
Turaki was speaking a day after Boko Haram leader Abubakar Shekau posted a video offering to release the girls in exchange for prisoners held by the government.
The video showed more than 110 girls sitting on the ground in a rural location. Though at least some of them are Christian, and Shekau described them as 'infidels', they were wearing full Islamic veils and singing and chanting Muslim prayers.
It was not clear when it was filmed or whether Shekau, who sat in front of a green backdrop holding an AK-47 during part of the video, was in the same location as the girls.
Those shown were among 276 abducted on April 14 from a secondary school in the northeastern village of Chibok, in a sparsely populated region near the borders with Cameroon, Niger and Chad. Some escaped, but about 200 are still missing. The group initially threatened to sell them into slavery.
Jonathan asked parliament on Tuesday for a six-month extension of a state of emergency in the northeastern states of Adamawa, Borno and Yobe due to persistent attacks by Boko Haram.
The emergency was declared last May and extended in November.
After being accused of a sluggish response to the kidnapping, the government has sent thousands of troops to the region, while the United States and Britain also have teams on the ground to help with the search.
The U.S. State Department said Washington had sent in military, law-enforcement and development experts.
"We have shared commercial satellite imagery with the Nigerians and are flying manned ISR (intelligence, surveillance and reconnaissance) assets over Nigeria with the government's permission," a U.S. official said.
U.S. State Department spokeswoman Jen Psaki said U.S. teams on the ground "are digging in on the search and coordinating closely with the Nigerian government as well as international partners and allies".
Britain's minister for Africa Mark Simmonds would travel to the Nigerian capital on Wednesday for talks on further assistance, the Foreign Office in London said.
Monday's video marked the first time the girls have been seen in captivity, though it remained unclear where they were being held and in how many groups.
A mother of one of the girls watched the video on television and spotted her daughter among the girls sitting on the ground, said Dumoma Mpur, parent-teachers' association chairman at Government Girls Secondary School in Chibok.
"The video got parents apprehensive again after watching it, but the various steps taken by the governments and the coming of the foreign troops is boosting our spirit," Mpur told Reuters.
Mpur said he was yet to see a soldier involved in the hunt in Chibok, though he said what looked like a surveillance jet and two helicopters were visible over the area.
Officials showed the video to a handful of other parents and a few of the escaped girls in Maiduguri on Tuesday but afterwards they made no comment.
Jonathan returned to Abuja on Tuesday from the Congo Republic, where he held talks with President Denis Sassou before a regional security summit in Paris on Saturday to discuss Boko Haram.