By Stephanie Nebehay
GENEVA, May 6 (Reuters) - The United Nations warned Islamist Boko Haram militants on Tuesday that there was no statute of limitations if they carried out their leader's threat to sell more than 200 Nigerian schoolgirls kidnapped last month.
Boko Haram leader Abubakar Shekau said in a video released on Monday that Allah (God) had told him to sell the girls taken by his fighters from a secondary school in the village of Chibok, in northeastern Borno state, on April 14.
"We warn the perpetrators that there is an absolute prohibition against slavery and sexual slavery in international law. These can under certain circumstances constitute crimes against humanity," U.N. human rights spokesman Rupert Colville told a news briefing in Geneva.
"That means anyone responsible can be arrested, charged, prosecuted, and jailed at any time in the future. So just because they think they are safe now, they won't necessarily be in two years, five years or 10 years time," he said.
He also urged Nigeria's federal and local authorities to work together to rescue the girls.
Local states have a lot of power and control over their territory, and the authorities in Borno are not of the same political party as the president, Colville said.
"So it is particularly important that there is close cooperation for the greater good, if you like, in this case, which is the release of these girls," he said.
Any buyer could also be held liable, Colville said, noting that enslaved girls are likely to be exposed to "continuous physical, psychological, economic and sexual violence" and that forced marriage can have a "devastating" impact on victims.
"The power differentials between girls and their 'spouses' is likely to undermine all autonomy, all freedom of will and expression of the girls. The situation they will be in will be tantamount to slavery, or slavery-like practices within the so-called marriage," he said.
U.N. High Commissioner for Human Rights Navi Pillay said after a visit to Nigeria in March that abuses by the security forces are boosting support for the group which has waged an increasingly bloody five-year-old insurgency in the north.
Pillay, a former U.N. war crimes judge, also said at the time that Boko Haram's actions were more and more monstruous.
She wrote to President Goodluck Jonathan on April 28 urging him to spare no effort to ensure the girls' safe return.
Any rescue attempt must be made in line with international human rights standards, Colville said, noting previous "allegations of excessive use of force by the Nigerian military in anti-Boko Haram operations".
Civilians should not be endangered, nor should there be summary executions or arbitrary detentions of suspects, he said.
(Editing by Louise Ireland)