Police in Nigeria have detained a dozen suspects linked to the discovery of "sleeper cells" operated by the Boko Haram Takfiri militant group in and around the capital, Abuja.

In a Saturday statement, the Department of State Services (DSS) said those arrested had traveled from the restive northeast of the country to the capital, which is located in central Nigeria, and were planning to carry out attacks in Abuja during the year-end festivities.

"Boko Haram ... has continued to establish and operate sleeper cells whose mandates are to conduct surveillance and carry out subsequent attacks," the DSS statement said.

The suspects were carrying out surveillance to “map out soft targets for attacks,” it added.

The statement also listed the names of 12 alleged Boko Haram members, including a Niger national, who had been arrested in Abuja and the neighboring towns between October 29 and December 1.

At least 20,000 people have been killed and more than 2.5 million made homeless since the beginning of the Boko Haram militancy in Nigeria in 2009. The militants have recently pledged allegiance to the Takfiri Daesh terrorist group, which is primarily operating inside Syria and Iraq.

Boko Haram attacks have spread from northeast Nigeria, its traditional stronghold, to the neighboring countries of Chad, Niger and Cameroon.

Nigeria, Chad, Niger, Cameroon, and Benin have set up a joint military force to eradicate the Takfiri militant group.