Nigeria's President Goodluck Jonathan on Thursday sacked his military top brass leading the fight against Boko Haram insurgents, following a court judgement that their appointment was illegal.

The heads of the country's army, navy and air force, who have been at the vanguard of efforts to end more than four years of Islamist violence, have all been replaced, according to his office.

There was no immediate indication that the decision was a direct result of dissatisfaction at their strategy against the militants, whose bloody insurgency has claimed thousands of lives since 2009.

Defence spokesman Chris Olukolade said the decision would not affect counter-insurgency plans.

"Military operation is a continuum," he told AFP. "The new military chiefs will be briefed adequately by their predecessors and the operation will continue as planned."

A retired, former senior military officer, added that the sackings would make little difference but instead called for a change of tactics.

"Boko Haram members are guerrilla fighters and Nigeria as of now is not prepared for this kind of security challenge," said the officer, who asked to remain anonymous.

"Terrorists always change their tactics because they are not fighting a conventional battle. So, the military also needs to change its tactics.

"It is not the change of military chiefs that matters in defeating insecurity in Nigeria."

Legal challenge

Human rights lawyer Festus Keyamo last year mounted a legal challenge to the officers' appointments, arguing they were not approved by the national assembly, as demanded by the constitution.

A judge in the Federal High Court in Abuja declared last July that the appointments violated section 18 of the Armed Forces Act 2004.

The law intends "to subject the armed forces to civil authority", Keyamo said, adding that neither the president nor the service chiefs were constitutionally superior to the national assembly.

In a January 7 letter to Jonathan and national assembly leaders, Keyamo gave a 14-day ultimatum to the government to comply with the court ruling.

Failure to do so would see him move the courts "to compel compliance", he added.

The service chiefs "were all appointed without the confirmation of the national assembly. Their appointments are therefore null and void ab initio (from the start)", he stated.

Keyamo said on Thursday that he had been "absolutely vindicated".

Renewed violence

Nigeria's military meanwhile said that Boko Haram fighters had been killed in clashes in the Banki area of northeastern Borno state, near the border with Cameroon.

"Many of the terrorists (Boko Haram) were killed in the fighting. I don't have a precise figure," said Olukolade, adding that two soldiers also lost their lives.

"But we are conscious of the fact that many of them were killed."

Borno state police commissioner Lawan Tanko confirmed the target was a police station in Banki and one police officer died.

Eye-witnesses said Boko Haram fighters also shot locals and burnt houses.

Police in the far north of Cameroon and the country's state radio reported earlier that about 15 people were killed, four of them soldiers, and the rest civilians.

Five civilians -- four Cameroonian and one Nigerian -- on the Cameroon side of the frontier were wounded by stray bullets.

The Boko Haram fighters were heavily armed and drove in pick-up trucks across the porous border from Amchide, prompting a firefight that lasted nearly four hours, the sources in Cameroon added.

A total of 19 people were killed on Tuesday when a car bomb ripped through a busy market in the capital of Borno state, Maiduguri.

The military blamed Boko Haram but the group has yet to claim responsibility.