A Department of State Services statement said it broke up a ring that had "perfected plans to attack the UK and American embassies and other Western interests" in Nigeria's capital, Abuja.
The statement said five suspects who had been based in Benue state and the Federal Capital Territory were arrested on March 25 and 26.
Another 20-year-old Boko Haram member was arrested March 22 in Yobe state and "confessed his involvement in executing the sinister activities of the group."
The US embassy did not immediately respond to a request for comment. The State Department on April 5 issued an updated travel warning for Nigeria, warning that Boko Haram has targeted government installations and other venues in the past in the Federal Capital Territory and elsewhere.
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"We are grateful for the support we receive from the Nigerian security authorities in protecting UK diplomatic staff and premises in Nigeria," a British government statement said.
"We are in regular contact with the Nigerian security authorities concerning potential threats to UK interests in Nigeria."
One faction of Boko Haram is allied with the Islamic State group.
Nigeria's president late last year declared the Boko Haram insurgency "crushed," but its fighters continue to threaten the vast region around Lake Chad in defiance of a multinational force. It has increasingly used children, especially girls, as suicide bombers.
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