The US has designated Nigeria’s Boko Haram and Ansaru militant groups as foreign terrorist organisations.
The state department described the move as “an important” step to help Nigeria “root out violent extremism”.
It means US regulatory agencies are instructed to block business and financial transactions with the groups.
Boko Haram wants to impose Islamic law in northern Nigeria, and has been blamed for thousands of deaths. Ansaru is seen as an off-shoot of Boko Haram.
Boko Haram began its insurgency in 2009, and targets both the military and civilians, including schools, and frequently clashes with the Nigerian armed forces.
Ansaru was formed in January 2012, though it rose to prominence only about six months later through the release of a video in which it vowed to attack Westerners in defence of Muslims worldwide.
The group has proved to be a formidable threat during its short existence, using dynamite to penetrate heavily-fortified compounds and taking foreigners hostage.
In a statement, the US state department said Wednesday’s “designations are an important and appropriate step”.
But it stressed that this was “only one tool in what must be a comprehensive approach by the Nigerian government to counter these groups”.
The decision means that it is now a crime under US law to provide material support to the two groups.
Boko Haram was earlier seen as an organisation which only posed a domestic threat – one reason why the US has not previously designated it as a terrorist organisation, the BBC’s Will Ross in Lagos reports.
But during the last three years – as its attacks have intensified – there have also been signs of a more international agenda, our correspondent says.
This included the 2011 raid on the UN building in the capital, Abuja, and also reports that Boko Haram has established links with al-Qaeda in the Islamic Maghreb and Al Shabab in Somalia.
The group has several sources of funding – including bank lootings and ransom payments – which are unlikely to be affected by the US decision, our correspondent adds.
Nigeria’s government said in June that Boko Haram and Ansaru were terrorist organisations, warning that anyone who helped them would face a minimum prison sentence of 20 years.
There have been allegations that some Nigerian politicians have supported Boko Haram.
There is as yet no evidence to suggest the ongoing Nigerian military campaign is succeeding against the group.
Civilians in the country’s north-east are killed by the group every week, and the army is frequently accused of carrying out human rights atrocities there, our correspondent says.
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