ABUJA (Reuters) - Nigerian President Goodluck Jonathan has declared the Islamist rebel force Boko Haram and splinter group Ansaru to be terrorist organizations, meaning anyone who supports them could face a 20-year-jail sentence.
The designation also means militants no longer must be prosecuted for specific crimes such as murder.
Jonathan's government had previously seemed reluctant to apply the label for fear of inflaming the situation but diplomats had raised concerns that Nigerian criminal laws were not suited to deal with the conflict.
His office made the announcement on Wednesday as the Nigerian military said it had detained 49 Boko Haram members during an offensive in the northeast, bringing the total captured to more than 150 in three weeks.
The president declared a state of emergency in three northeastern states last month and sent troops to try to break the four-year-old insurgency that has killed thousands.
Under Nigeria's 2011 Terrorism Prevention Act, anyone who incites a terrorist act over the Internet or through any other media, or gives weapons or any assistance to a terrorist group is liable for a minimum 20-year prison sentence.
"Any persons associated with the two groups can now be legally prosecuted and sentenced to penalties specified in the Act," presidential spokesman Reuben Abati said in a statement.
Before Boko Haram and Ansaru received the terrorist label, members could only be prosecuted for specific crimes like murder or fire arms possession - a situation that meant hundreds of Boko Haram detainees considered too dangerous to release were being held without charge often for years at a time.
It was not clear how this would affect an amnesty being offered to members who turn themselves in. In a move meant to appease moderate supporters, Nigeria this week released all women and children being held for links to the group.
Ansaru, a smaller outfit that grew out of Boko Haram and is thought to have direct links to al Qaeda's North African wing, has been blamed for a number of kidnappings of Westerners in Nigeria, most of whom they killed in captivity. Britain last year declared it a terrorist group.
The military offensive in the semi-desert along the borders with Cameroon, Chad and Niger is Jonathan's boldest effort yet to crush the Islamists. Security sources say soldiers from Niger and Cameroon are also involved.
Boko Haram leader Abubakar Shekau, whose movement wants to revive an old Islamic caliphate in religiously mixed Nigeria, said last week that the operation was failing.
On Monday, the United States, which has had Shekau on its terrorist blacklist since last year, offered a $7 million reward for information leading to his capture. Nigeria has had a $1.8 million bounty on Shekau and 19 other Boko Haram leaders since November