ABUJA (Reuters) - Nigeria's government signaled a willingness on Tuesday to negotiate with Islamist militants holding more than 200 schoolgirls, a month after the kidnapping that has provoked global outrage.

"The window of negotiation is still open," Minister of Special Duties Tanimu Turaki told Reuters by telephone.

He 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.

Senior officials say the government is exploring options and has made no commitment to negotiations for the release of the girls, and Turaki declined to comment on possible talks over the kidnapping itself.

Instead, he referred to an amnesty committee that he heads, set up by President Goodluck Jonathan last year to talk to the Boko Haram militants behind a five-year-old insurgency.

The committee's initial six-month mandate expired without holding direct talks with the rebels, though it has spoken to them through proxies. It has since been replaced by a standing committee empowered to conduct talks, officials said.

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.

The United States has sent military, law-enforcement and development specialists.

Two U.S. officials said on Tuesday a mix of manned and unmanned American surveillance aircraft were being used to aid the search for the missing girls. One U.S. official identified the drone as a Global Hawk, which is a high-altitude, unmanned spy plane manufactured by Northrop Grumman Corp.

The Boko Haram video showed more than 110 girls sitting on the ground in a rural location, the first time they have been seen in captivity.

Although 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 or where the video 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 village of Chibok, Borno state, 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.

STATE OF EMERGENCY

Jonathan returned to Abuja on Tuesday from the Congo Republic, where he held talks with President Denis Sassou ahead of a regional summit in Paris on Saturday.

He 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.

Yobe state Governor Ibrahim Gaidam rejected the proposal on the grounds that local people had suffered under the emergency and this harmed the government's counter-insurgency strategy.

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.

Britain's minister for Africa, Mark Simmonds, will travel to the Nigerian capital on Wednesday for talks on further assistance, the Foreign Office in London said.

Borno state Governor Kashim Shettima said 77 of the girls in the video had been identified by parents, fellow students and girls who escaped the abductions.

"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," said Dumoma Mpura, a leader at the girls' boarding school.