Hundreds of bodies remain strewn in the bush in Nigeria amid an ongoing attack described by Amnesty International as the "deadliest massacre" by Boko Haram.

Mike Omeri, the government spokesman, said fighting continued on Friday for Baga, a town on the border with Chad where Boko Haram fighters seized a key military base on January 3 and attacked again on Wednesday.

"Security forces have responded rapidly, and have deployed significant military assets and conducted air strikes against militant targets," Omeri said in a statement.

District head Baba Abba Hassan said most victims are children, women and elderly people who could not run fast enough when fighters drove into Baga, firing rocket-propelled grenades and assault rifles on town residents.

"The human carnage perpetrated by Boko Haram terrorists in Baga was enormous," Muhammad Abba Gava, a spokesman for poorly armed civilians in a defence group that fights Boko Haram, told The Associated Press news agency.

He said the civilian fighters gave up on trying to count all the bodies.

"No one could attend to the corpses and even the seriously injured ones who may have died by now," Gava said.

An Amnesty International statement said there are reports the town was razed and as many as 2,000 people killed.