Gunmen have killed at least 35 people in three villages in Nigeria's northeastern Borno state, a military source and residents said.

Dozens of men dressed in military uniform stormed Gumushi, Amuda and Arbokko in all-terrain trucks and on motorcycles on Wednesday, near the border with Cameroon.

"The insurgents hurled petrol bombs into homes, setting them ablaze and shot residents as they tried to escape," a military officer told the AFP news agency about the attack on Gumushi.

He blamed the violence on the armed group Boko Haram, which has stepped up deadly raids in northeast Nigeria in recent months, pillaging and burning entire villages and killing residents as part of its five-year-old campaign to establish an Islamic state in the north.

News of the latest attacks came hours after Nigerian President Goodluck Jonathan said he had ordered a "total war" against the group, which last month abducted 276 schoolgirls in Borno. Nearly 220 of them are still missing.

Jonathan said that he had authorised security forces to use "any means necessary under the law".

The northeastern state has been under a state of emergency since 2013.

Al Jazeera's Yvonne Ndege, reporting from the capital Abuja, said the president's speech had no information on what his government was doing to bring the abducted girls back home.

The phrase "total war" was used by Idriss Deby, the president of neighbouring Chad, following a meeting of West African countries in Paris in mid-May, designed to define a common strategy to fight Boko Haram.

Nigeria has been battling Boko Haram for years without success, and has had to seek assistance from world powers such as the US, France, Britain and China to free the abducted girls.

Source: Al Jazeera and agencies