Boko Haram fighters from Nigeria have kidnapped at least 60 people in a new attack in northern Cameroon in which some people were killed.

The Islamist militants "burst into two villages in the Tourou area... They torched houses and left with around 60 people. Most of them were women and children," a police officer said.

He said the cross-border attack had "left some people dead" without giving an exact toll, adding that the Cameroon army had "launched an operation" in the wake of the assault.

The kidnapping was the biggest in Cameroon by the Islamists who have staged a series of attacks in the country in recent months and escalated their bloody insurgency in their stronghold in northeastern Nigeria.

The assault was launched after neighbouring Chad deployed troops to combat Boko Haram both in Cameroon and Nigeria.

In Nigeria the Chadian troops are seeking to recapture the strategic city of Baga, on the shores of Lake Chad, which straddles the borders of Chad, Nigeria, Niger and Cameroon and which fell to the Islamists early this month. Many say the assault on Baga could be Boko Haram's deadliest.

Satellite pictures released by Amnesty and Human Rights Watch showed widespread destruction with around 3700 buildings in Baga and nearby Doron Baga damaged or destroyed.

Amnesty says as many as 2000 civilians may have been massacred, but Nigeria's army objected to the "sensational" claims and said that the death toll in Baga was about 150.

Some 400 Chadian army vehicles arrived in the Cameroonian border town of Kousseri on Saturday, and Chadian President Idriss Deby said they were "operational" as of Sunday.

Boko Haram last Monday launched an offensive against a Cameroonian military base in Kolofata, also in the far north of the country, in which 143 "terrorists" and one Cameroonian soldier were killed, according to Cameroon.