WASHINGTON — Two Islamist groups in Nigeria have been added to the State Department's list of foreign terrorist organizations for killing thousands of people and threatening Westerners in West Africa, U.S. officials said Wednesday.

Boko Haram and a splinter group, Ansaru, were named to the federal roster of terrorist groups after U.S. officials determined that they had received training and some financing from the Al Qaeda affiliate in North Africa.

The 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 counter-terrorism advisor, said in a statement.

The listing makes it a federal crime to knowingly provide support to the groups. It also blocks them from the U.S. financial system and enables banks to freeze their U.S. assets. The State Department previously had named three of Boko Haram's leaders to its global terrorism sanctions list.

Experts don't believe either group has strong enough U.S. links to be greatly harmed by the financial penalties. Both are focused on Nigeria and have their roots in economic and political grievances among the country's mainly Muslim north.

Boko Haram, whose name means "Western education is a sin," has attacked schools, churches, security forces and journalists in its campaign to impose sharia, or Islamic law, in northern Nigeria.

The group has said it wants to "eradicate Christians" from certain parts of Nigeria, Africa's most populous nation. Roughly half the people in Nigeria identify themselves as Christian.

Boko Haram is blamed for a suicide bombing at the United Nations office in Abuja, the Nigerian capital, that killed 21 people in 2011, and the kidnapping of seven French citizens in neighboring Cameroon in February. In September, it was suspected in an attack on a college dormitory in northern Nigeria in which at least 40 people were killed.

Ansaru has repeatedly attacked Nigerian troops and kidnapped foreigners working in Nigeria, one of the world's major oil producers.

U.S. officials acknowledged that Boko Haram "remains primarily a Nigerian organization" but said the groups pose a sufficient enough threat to the United States that the terrorist designations were warranted.

"There is a very large American population in Nigeria and a lot of U.S. investment in Nigeria," said an official who briefed reporters on condition he not be identified. "Threats to Nigeria automatically impact the U.S. economy and American citizen interests."

The designation also was influenced by Nigeria's status as the economic engine of West Africa, and a burgeoning power that contributes nearly 5,000 soldiers and police to international peacekeeping operations. The Pentagon has conducted training missions with Nigerian forces and occasionally has sent vessels to patrol the Gulf of Guinea, through which nearly 30% of U.S. oil imports pass.

But Nigerian security forces also have been blamed for widespread abuses as they battle the northern insurgency.

Last month, Amnesty International said that more than 950 people died in Nigerian military custody in the first half of 2013, and that most of the deaths took place in prisons where suspected Boko Haram members are held. The human rights group cited interviews with former prisoners that suggested many detainees may have been shot to death.

U.S. officials say they have raised concerns about possible abuses in meetings with Nigerian leaders.

shashank.bengali@latimes.com