A Nigerian air force jet has been missing for two days after disappearing in the country's far northeast, where troops are fighting Boko Haram insurgents, the military has said.

A search and rescue operation was under way for the Alpha jet which was on a routine operational mission on Friday over Adamawa state when contact was lost with the aircraft, the Nigerian armed forces said in a statement.

The aircraft, with two pilots on board, left its base in Yola, Adamawa, at about 0945GMT on Friday and had been expected back by midday the same day.

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"Since then all efforts to establish contact with the aircraft have not yielded any positive result," military spokesman Chris Olukolade said in a statement, without providing further details.

Boko Haram has seized territory in Adamawa, Borno, and Yobe states in recent weeks, raising fears about a loss of government control in the far northeast and putting the military under pressure to put an end to the five-year-long armed movement.

Nigeria's military, backed by war planes, has been fighting to push back recent advances by Boko Haram into the north of Adamawa state and also towards Maiduguri, the state capital of neighbouring Borno state. The army said it beat back an attack outside the city of Maiduguri on Friday.

Boko Haram, which has killed thousands since it launched its anti-government rebellion in the northeast in 2009, has in recent weeks captured and held a string of towns and villages in the region, a departure from its usual hit-and-run tactics.

Its leader, Abubakar Shekau, is apparently trying to follow the example of Islamic State in Iraq and Syria, which has declared its own caliphate. Nigeria's government has vowed to take back the seized territory from Boko Haram.

Source: Agencies