Chadian troops have crossed into northeastern Nigerian town of Gambaru to launch a ground offensive for the first time against Boko Haram fighters that control the town bordering Cameroon, Chadian military sources have said.

Fighting between the two sides began after armoured vehicles and soldiers from Chad entered the strategic Nigerian town on Tuesday in an increasingly regional conflict.

"Our troops entered Nigeria this morning. The combat is ongoing," one of the sources at Chad's army headquarters told the Reuters news agency.

Chadian forces have also taken up position close to Boko Haram strongholds along Nigeria's border with Niger.

"A contingent of about 400 vehicles and tanks is stationed between Mamori and Bosso," Niger's private radio Anfani reported.

The ground offensive followed days of intense combat between Boko Haram fighters and Chadian forces in Cameroon, during which Chad's air force carried out strikes on the rebels, Chadian and Cameroon military sources said.

Boko Haram fighters had launched attacks across the border bridge from Gambaru into Cameroon, the sources said.

Nigeria's military said the country's sovereignty was not compromised despite the presence of Chadian ground troops on its territory.

"Nigeria's territorial integrity remains intact," Defence spokesman Chris Olukolade said, claiming national forces had "planned and are driving the present onslaught against terrorists from all fronts in Nigeria, not the Chadian forces".

Chad has deployed some 2,500 troops as part of a regional effort to take on the armed group that has waged a rebellion in Nigeria's north, killing an estimated 10,000 people last year. The group says it wants to establish an Islamic state in Africa's biggest oil producer.

Major supply route

The road from Gambaru to Fotokol in Cameroon is one of Boko Haram's major supply routes. It has been hampered since Cameroon deployed special forces to the area in mid-2014, leading to fierce fighting in the area.

The Nigerian government said on Monday that Gambaru alongside several other towns in the region including Mafa, Mallam Fatori, Abadam and Marte had been liberated from Boko Haram.

In a further sign of mounting international action to combat the militant group, France said on Tuesday that French military aircraft are carrying out surveillance missions to help countries bordering Nigeria tackle Boko Haram.

The African Union (AU) has authorised a force of 7,500 troops from Nigeria, Chad, Cameroon, Niger and Benin to fight the rebels.

Source: Agencies