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Nigeria has officially banned two militant Islamist groups, warning that anyone who helps them will face a minimum jail sentence of 20 years.

President Goodluck Jonathan declared Boko Haram and Ansaru to be terrorist groups, his office said.

The army has been waging an offensive against the militants in their northern strongholds since a state of emergency was declared last month.

The insurgency has killed about 2,000 people since 2009.

The activities of both Boko Haram and Ansaru would now fall under the Terrorism Prevention Act, Mr Jonathan's office said, in a statement.

It means that "any person who knowingly, in any manner, directly or indirectly", offers support to Boko Haram and Ansaru would be jailed for "not less than 20 y