The United Nations has officially declared Boko Haram a terrorist group linked to al Qaida.

The world body's Security Council also imposed sanctions against the Islamist extremists who have carried out a wave of deadly attacks and the abduction of nearly 300 schoolgirls in Nigeria.

Samantha Power, US ambassador to the UN, welcomed the council's action, calling it "an important step in support of the government of Nigeria's efforts to defeat Boko Haram and hold its leadership accountable for atrocities".

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Nigeria, which is serving a two-year term on the Security Council, asked its committee monitoring sanctions against al Qaida to add Boko Haram to the list of al Qaida-linked organisations subject to an arms embargo and asset freeze.

The 14 other council members had until Thursday night to object: none did. The group was then added to the UN sanctions list under the name Jama'atu Ahlus-Sunna Lidda'Awati Wal Jihad, with Boko Haram as an alias.

Australia's UN ambassador Gary Quinlan said there was "very clear evidence" Boko Haram members trained with al Qaida in the Islamic Maghreb.