PHOTO: Nigerian forces in Baga in 2013 (Utomi Ekpei/Getty/AFP)

Up to 2,000 Nigerians are feared dead in a week of attacks by the Islamist group Boko Haram.

Hundreds of bodies remain strewn in the bush as fighting continues around the town of Baga, on the border with Chad, where insurgents seized a key military base last Saturday.

A Government spokesman said Nigerian forces, supported by airstrikes, are counter-attacking.

District head Baba Abba Hassan said most of the victims are children, women and elderly people who could not flee when insurgents drove into Baga, firing rocket-propelled grenades and assault rifles on residents.

Muhammad Abba Gava, a spokesman for a defence group that fights Boko Haram, said, “No one could attend to the corpses and even the seriously injured ones who may have died by now.”

Amnesty International, citing reports that Baga had been razed, said up to 2,000 people had been killed.

In the previous bloodiest day in the fighting between Boko Haram and the Nigerian military, soldiers gunned down unarmed detainees freed after an attack last March on Giwa military barracks in the city of Maiduguri. Amnesty said satellite imagery indicated more than 600 people died.

The three northeastern states worst hit by Boko Haram asked the central government for more troops earlier this week. Around 1.5 million people have been displaced by the violence, and more than 11,000 people are believed to have been slain in 2014.

Boko Haram, a Sunni Islam fundamentalist group, was founded in 2002. It turned to violence in 2009 after detention of its members, with a rapid escalation after its leader died in custory. It now controls an area of northeastern Nigeria which is about the size of Belgium.