Nigeria detained a Russian cargo plane and its French-speaking crew on Saturday after it made an unauthorised landing in the northern city of Kano with military hardware bound for neighbouring Chad, a security source said.
An air force spokesman confirmed the plane had been detained but gave no further details.
The plane had been travelling from Bangui, the capital of Central African Republic, to the Chadian capital N'Djamena but was forced to make an emergency landing in Kano because N'Djamena airport was closed, the security source said.
He said after the five crew members were detained, inspectors found two helicopters, a bullet-proof jeep and boxes of what were suspected to be military hardware.
The Russian embassy in Abuja did not immediately respond to a request for comment.
It was not clear what the shipment was for. Chadian fighters have been involved in Central African Republic's conflict since Muslim Seleka rebels seized the southern capital Bangui in 2013.
Mysterious arms shipments have caused controversy in Nigeria before. A Nigerian court sentenced an alleged member of Iran's Revolutionary Guard and a Nigerian accomplice to five years in prison in May last year over an illegal shipment of mortars and rockets seized in the main port of Lagos in 2010.