The United States on November 13, 2013 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. (HO/AFP/Getty Images)

The United States formally designated the Nigerian Islamist militant groups Boko Haram and Ansaru as foreign terrorist organizations Wednesday, making it a crime to provide them with material support.

The White House directed U.S. agencies to block financial transactions with the two groups, which it says are responsible for thousands of deaths in northeastern and central Nigeria, including attacks on churches and mosques and a 2011 suicide bombing of the U.N. building in Abuja, Nigeria’s capital.

“By cutting these terrorist organizations off from U.S. 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,” Lisa Monaco, President Obama’s top homeland security and counterterrorism adviser, said in a statement.

Boko Haram, whose name means “Western education is a sin” in the Hausa language, is seen along with other splinter groups as the biggest security threat facing Nigeria, Africa’s most populous country and top oil exporter.

U.S. lawmakers have been pushing for the group to be formally designated a foreign terrorist organization.

“What these murderers have brought to Nigeria and surrounding countries is misery and death with no redeeming outcome,” said Rep. Christopher H. Smith (R-N.J.), who chaired a congressional hearing on Boko Haram on Wednesday. Smith visited Nigeria in September.

U.S. officials said instability in Nigeria was of direct concern to the United States, noting that violence also discourages investment and that it has spread to Nigeria’s neighbors.

“These groups attack the Nigerian government, they attack the military, they attack ordinary Nigerians of all walks of life,” Assistant Secretary of State Linda Thomas-Greenfield told the hearing.

U.S. officials also cited reports that Nigerian security forces have violated human rights standards while fighting Boko Haram.

A senior administration official said that it was not immediately clear what assets Boko Haram and Ansaru held and that the U.S. Treasury needed the official designation in place before it could determine their holdings.

The official told reporters on a conference call that Washington had worked with the Nigerian government in making the designation.

Also Wednesday, the United Nations’ humanitarian agency said that Nigeria’s army offensive against Islamist militants in the country’s northeast has pushed nearly 40,000 refugees over its northern border into Niger, in a drive that is straining food supplies in the drought-prone country.

— Reuters