Boko Haram militants and the Nigerian military may have committed war crimes and crimes against humanity in the insurgency raging in the country's north, Amnesty International says.
The human rights monitor made the call as it said that more than 1500 people had been killed during increased violence in northeast Nigeria in the first three months of 2014 - the highest figure yet from organisations tracking the conflict.
Nigeria's emergency relief agency NEMA said last Tuesday that more than 1000 had been killed and 250,000 were displaced in the same period. Human Rights Watch said on March 14 that 700 people had died since the start of the year.
Amnesty, which said more than half of the victims were civilians, expressed particular concern about claims that hundreds of Boko Haram suspects were summarily executed after a jail break on March 14.
The group said the "alarming" increase in violence in Nigeria's northeast had turned the crisis into "a situation of non-international armed conflict in which all parties are violating international humanitarian law".
"We urge the international community to ensure prompt, independent investigations into acts that may constitute war crimes and crimes against humanity," said Africa research and advocacy director Netsanet Belay.
Attacks on remote, rural villages, a boarding school, churches and key military installations this year have piled pressure on Nigeria's government about its strategy to end the violence.
The military says a troop surge to three northeastern states under emergency rule since last May has been effective, cutting off the insurgents' escape routes and depriving them of arms and ammunition.
But Amnesty said a March 14 attack on the Giwa military barracks in the capital of Borno state, Maiduguri, was a "tipping point" because of the "brutal crackdown" on Boko Haram suspects held at the facility that followed.
There was "credible evidence" that more than 600 people, mostly unarmed recaptured detainees, were summarily killed across Maiduguri as the military regained control, Amnesty said.
Witnesses said the military shot and killed 56 escaped prisoners on land behind the city's university.
In a separate incident, nearly 200 others captured by civilian vigilantes were handed to the military and made to lie down.
"The soldiers made some calls and a few minutes later they started shooting the people on the ground," a witness said.
In both cases, the detainees reportedly claimed they were not militants, Amnesty said.
Amnesty has also called for the African Union, the Economic Community of West African States (ECOWAS) and the AU's Peace and Security Council, which Nigeria will chair from April, to help bring an end to the conflict.
