Nigeria's spy agency says it thwarted a plot by militant Islamist group Boko Haram to attack the UK and US embassies in the capital Abuja.
Six people were arrested last month in connection with the plot, the Department of State Services (DSS) added.
There has been no independent confirmation of the agency's claim.
Boko Haram, which is linked to the Islamic State group, has carried out an insurgency in Nigeria since 2009.
At least 21 people were killed when it carried out a car bomb attack on the UN headquarters in Abuja in 2011.
In a statement, the DSS said five of the men had "perfected plans" to attack the embassies.
They were arrested on 25 and 26 March in Abuja and central Benue state, the DSS added.
A sixth person was arrested on 22 March in north-eastern Yobe state, and he confessed to being part of the group's "sinister activities", the DSS said.
The US embassy in Abuja said it appreciated the DSS's efforts in "fighting terrorism", and the US and Nigeria had a "strong partnership in countering violent extremism".
A UK government spokesman said: "We are grateful for the support we receive from the Nigerian security authorities in protecting UK diplomatic staff and premises in Nigeria. We are in regular contact with the Nigerian security authorities concerning potential threats to UK interests in Nigeria."
It is unclear why the DSS released details of the alleged plot a few weeks after the arrests. However, the BBC's Chris Ewokor in Abuja says this is not unusual, as the agency first assesses whether releasing such information will jeopardise investigations.