Suspected Nigerian Boko Haram militants have carried out an attack on Chad overnight, the first such assault on Chadian soil, security sources say.

Officials said the fighters crossed Lake Chad by motorboat and attacked a village on the shores of the lake.

The Islamist militants were pushed back by Chadian troops after killing several people, residents said.

Chad recently joined Nigeria, Niger, and Cameroon in a military coalition against Boko Haram.

The jihadists, who want to create their own Islamic caliphate in Nigeria, have killed thousands and forced millions to flee their homes in north-eastern Nigeria since 2009.

'Homes torched'

Residents said around 30 militants attacked the village of Ngouboua, torching two-thirds of its homes.

"They came on board three canoes and succeeded in killing about 10 people before being pushed back by the army," one resident told Reuters.

The number of casualties was not confirmed but Chadian officials told the BBC that a local chief had been killed during the attack.

Chadian military aircraft carried out air strikes against the militants, destroying their vessels, officials said.

The Islamist militant group has intensified its attacks against Nigeria's neighbours in recent weeks, forcing thousands of residents to flee their homes.

Approximately 7,000 people fled to Ngouboua after Boko Haram attacked the Nigerian border town of Baga in January.

The militants have continued their insurgency in north-eastern Nigeria - at least 21 people were killed on Thursday in two separate attacks in Borno State, witnesses told AFP.

Militants killed 12 people in the village of Akida and nine in the village of Mbuta, community leader Mustapha Abbagini said.

Approximately 3.2 million people in Nigeria have fled their homes as a result of the Islamist insurgency, officials say, with many living in makeshift camps.

Nigeria launched an investigation on Tuesday after reports of rapes, child trafficking and other abuses in the camps.

The conflict with Boko Haram has forced a postponement of Nigeria's presidential and parliamentary elections from 14 February to 28 March.