Cameroon says its military has freed a German hostage kidnapped in Nigeria by the Islamist militant group Boko Haram.

Nitsch Eberhard Robert, a teacher, was abducted in July 2014 in Nigeria's Adamawa State by the group, Cameroon's government said.

He was freed in a "special operation of the Cameroon armed forces and security services of friendly countries".

Boko Haram has seized control of towns and villages in north-east Nigeria and launched raids into Cameroon.

It is not clear which countries contributed to the operation to free Mr Robert.

Germany's Foreign Ministry spokesman Martin Schaefer told the AP news agency that the man was now safely in Cameroon's capital.

He said a crisis team had been working on the case for months and thanked the Cameroon government for its "very good and trusting" co-operation.

Cross-state threat

Boko Haram, already dominant in north-east Nigeria, has begun threatening neighbouring nations.

Suspected Boko Haram militants were believed to have kidnapped dozens of people in a cross-border raid near Mokolo city in Cameroon's Far North region on Sunday.

Some were later freed as Cameroon's forces pursued the attackers who were fleeing back to Nigeria.

Previous kidnappings in Cameroon blamed on Boko Haram have been more targeted - with high-profile people or foreigners reportedly taken for ransom.