Gunmen in south-eastern Nigeria have attacked a US convoy, killing four people, local police say.

They say two of the victims of Tuesday's attack in the Anambra state were US consulate employees, while the other two were police officers.

The attackers kidnapped three other people, and set their vehicle on fire.

Washington says no US citizens were in the convoy, which was travelling in the state plagued by violence and a separatist insurgency.

Nigerian police say the attack happened on the Atani-Osamale road in Ogbaru region.

Police spokesperson DSP Ikenga Tochukwu says security forces were currently carrying out a rescue and recovery operation.

In a statement to the BBC, the US confirmed that "there was an incident on 16 May in Anambra state", adding that Washington was working with Nigerian security services to investigate the attack.

"The security of our personnel is always paramount, and we take extensive precautions when organising trips to the field," the US state department said.

The group has so far made no public comments on the issue.

The Nigerian authorities often blame violent attacks in the region on the Indigenous People of Biafra (Ipob) movement, which is fighting for a breakaway state in the south-east of the country.

Anambra and other parts of the south-east have seen a sharp rise in attacks on security forces since Ipob launched an armed wing in December 2020.