Two kidnapped US embassy staff have been rescued unharmed in Nigeria, say police, days after seven other people travelling in the same convoy were killed.

The attack happened on Tuesday in the south-eastern district of Ogbaru, which has been under curfew for the past year because of safety concerns.

A local official says the assailants shot at the vehicles and torched them, burning the dead beyond recognition.

It is not clear who the gunmen were.

Some officials are blaming Igbo separatists fighters from the Indigenous People of Biafra (Ipob) group, who are known to operate in this region. Ipob has not commented on the allegations.

President Muhammadu Buhari, who leaves office later this month after pledging and failing to end Nigeria's security crisis, says his government is "committed to fishing out" the culprits behind Tuesday's attack.

No US citizen was among the victims and there are "no indications at this time that it was targeted against our mission", said Secretary of State Antony Blinken.

Of the seven people killed on Tuesday, three were Nigerian staff at the US embassy and four were security escorts. The two people who were abducted, then released, have not been publicly identified by police.

"Operations are still ongoing and further details shall be communicated," says Anambra state police spokesman Ikenga Tochukwu.