Nigerian militant group Ansaru says it kidnapped seven foreign workers in a raid that saw a security guard killed.
Italian, Filipino, British, Greek and Lebanese workers are thought to be among those being held after the attack in the northern state of Bauchi.
They were working on a construction project when the militants attacked on Sunday, reports say.
Ansaru, an Islamist group, emerged last June and recently carried out a raid in which it kidnapped a French national.
The group announced it was behind Sunday's kidnappings in an email sent to journalists, saying it had "the custody of seven persons, which include Lebanese and their European counterparts" working with construction firm Setraco.
On Monday, the UK Foreign and Commonwealth Office would still not confirm or deny reports that a Briton was among those held.
The kidnapping of foreigners and wealthy Nigerians is common in the oil-rich Niger Delta region, in the south of the country, and has become a lucrative trade for criminals.
Oil workers and other foreign nationals are often targeted because companies pay high ransom money to secure their employees' release, correspondents say.
But the seizure of seven foreign workers in the north of the country makes Sunday's raid one of the most significant in that region in recent memory.
The raid was preceded by an attack on the local police station, when two vehicles were blown up, in the town of Jama'are, some 125 miles (200km) north of the state capital, Bauchi.
The attackers then moved on to a camp belonging to Setraco, killing a guard and seizing the workers. Setraco is currently expanding a major road in the area.
The state capital has itself been attacked several times by Boko Haram, which wants to impose Sharia (Islamic law) across Nigeria.