WASHINGTON - The United States on Wednesday blacklisted Nigeria’s radical Islamist Boko Haram network and an offshoot known as Ansaru as terror groups, bowing to months of pressure to move against the militants.

“These designations are an important and appropriate step, but only one tool in what must be a comprehensive approach by the Nigerian government to counter these groups ... to help root out violent extremism,” the State Department said in a statement.

The Islamist insurgency by the shadowy group has claimed thousands of lives since 2009 mainly in northeast and central Nigeria and caused international concern over its potential to spread across porous borders into other nearby safe havens.

In July, the State Department’s Rewards for Justice program offered a $7 million reward for information leading to the arrest of Boko Haram leader Abubakar Shekau, but raised eyebrows by stopping short of designating the group as a foreign terrorist organization.

“In the last several years, Boko Haram and Ansaru have been responsible for thousands of deaths in northeast and central Nigeria, including dozens of attacks on churches and mosques, targeted killings of civilians, and the 2011 suicide bombing of the United Nations building in Abuja that killed 21 people and injured dozens more,” White House homeland security advisor Lisa Monaco said in a statement.

“By cutting these terrorist organizations off from US financial institutions and enabling banks to freeze assets held in the United States, these designations demonstrate our strong support for Nigeria’s fight against terrorism and its efforts to address security challenges in the north.”

US officials accuse Boko Haram of links to Al-Qaeda in the Islamic Maghreb, while Ansaru is a splinter faction that earlier this year kidnapped and executed seven foreign construction workers.

Emergency rule in the northeast has largely pushed Boko Haram fighters from urban areas into the countryside over the last six months, but attacks have continued unabated.

The three Nigerian states under special measures — Yobe, Borno and Adamawa — share frontiers with Niger, Chad and Cameroon and the military has said that insurgents have struck in Nigeria, then fled across the porous borders.

“All of our assistance to Nigeria stresses the importance of protecting civilians and ensuring that human rights are respected. That assistance and these designations demonstrate US support for the Nigerian people’s fight against Boko Haram and Ansaru,” the State Department said.

Both groups were officially designated as Foreign Terrorist Organizations which will bar any Americans from assisting them as well as freezing all their assets in the United States.

President Barack Obama met Nigerian President Goodluck Jonathan in September and urged him “to pursue a comprehensive counterterrorism approach that uses law enforcement tools effectively, creates economic opportunity, and ensures that human rights are protected and respected,” Monaco said.

US ‘security interest’ in stable Nigeria

Roughly translated, Boko Haram means “Western education is sin,” and the insurgents have been blamed for a series of bloody attacks on schools, killing dozens of children, with some analysts suggesting the group has selected shocking targets to generate attention.

Boko Haram has said it is fighting to create an Islamic state in Nigeria and Shekau has repeatedly called for a nation governed by sharia, or Islamic law.

Local and Western analysts have long argued that improving living conditions in the mainly Muslim north is key to curbing the insurgency.

Boko Haram is blamed for indiscriminate attacks in Benisheikh, Nigeria in September 2012 in which some 160 people were killed, but was also said to be behind the suicide bombing of a UN building in Abuja in August 2011.

Ansaru has focused attacks on Nigerian military and Western targets, kidnapping several foreigners living and working in the country.

In November 2012, it raided a police station in Abuja, killing Nigerian police officers and freeing detained terrorists from prison, the State Department said.

Representative Ed Royce, chairman of the House Foreign Affairs Committee, welcomed the move saying the United States had “a security interest” in a stable Nigeria.

“Once believed to be strictly local, its fighters are increasingly linking up with other al-Qaeda linked groups in the region,” Royce said in a statement.