Boko Haram Islamists have attacked a military base in the northeastern Nigerian city of Maiduguri and freed dozens of insurgents from custody.
Before storming the Giwa barracks, a notorious prison for Islamist fighters, the gunmen opened fire in a residential neighbourhood and razed many homes, witnesses said on Friday.
The attack is another blow to the military's campaign against the extremist group, which has been blamed for thousands of killings since 2009 in Africa's most populous country and top oil producer.
Gunmen hurled explosives into the compound, causing several buildings to catch fire, witnesses said.
A military source who requested anonymity told AFP news agency the insurgents forced their way inside the barracks prison and "freed dozens of their members".
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Asked about the report, defence spokesman Chris Olukolade told AFP "many insurgents are in flight. Whether they were those who staged the attack or those who were held prisoner is not yet known."
In a separate statement, Olukolade said the insurgents have been hit hard by military bombardments in recent weeks and Boko Haram attacked the barracks "to boost their depleted stock of fighters".
The insurgents have previously carried out a number of prison breaks but this appeared to be the most dramatic in many months.
The northeast has been under a state of emergency since May when the military launched a major offensive aimed at crushing the Islamist uprising.
Defence officials have sought to portray the operation as a success but the violence has continued unchecked.
Human Rights Watch on Friday said Boko Haram had carried out more than 40 attacks already in 2014, with more than 700 people killed, one of the highest rates of killing since the uprising began.
