Boko Haram crisis: Chad's troops enter Nigeria
Chadian troops have entered Nigeria to join the battle against militant Islamist group Boko Haram.
Armoured vehicles and infantry crossed a bridge from Cameroon following air strikes and mortar attacks on Boko Haram positions, officials say.
Fighting focused on the key north-eastern town of Gamboru, Nigerian security spokesman Mike Omeri said.
Chad's deepening involvement shows how the conflict with Boko Haram is taking a regional dimension.
Last week, Chadian troops reportedly moved into Malumfatori, a Nigerian town which lies near the borders of Chad and Niger, after a ground and air assault against the militants.
Chadian forces have also massed near the town of Diffa in Niger, Reuters news agency quotes military sources in Niger as saying.
Boko Haram leader Abubakar Shekau has threatened to create a caliphate, incorporating parts of Nigeria, Niger, Chad and Cameroon.
The African Union has responded by backing plans to establish a 7,500-strong regional force to fight the group.
The Chadian contingent of about 2,000 troops crossed the frontier without a shot being fired, AFP news agency reports from the scene.
Chad warplanes had earlier carried out air strikes for about an hour.
Mr Omeri told BBC Focus on Africa that Chad's intervention signalled the "intensification" of efforts by neighbouring states to defeat Boko Haram.
"We should expect more cooperation, more collaboration and more coordination," he said.
Chadian military sources told Reuters that their troops had clashed with the militants in Gamboru, a small town separated from Cameroon by a river and used by the militants to launch cross-border raids.
"Our troops entered Nigeria this morning. The combat is ongoing," it quotes a source as saying.
On Monday, Nigeria's army said it had recaptured Gamboru.
Most of its residents had fled after Boko Haram seized it last year, destroying most of the town and killing hundreds of people.
Nigeria's military has been widely criticised for failing to curb the six-year insurgency, which has displaced some 1.5 million people.
It is under increasing pressure to regain territory ahead of the 14 February presidential election, amid concerns that the insurgency will prevent many Nigerians in the north-east from voting.
Boko Haram controls territory the size of Belgium, mostly in Borno state, according to Associated Press news agency.
In April, it caused international outrage by abducting more than 200 schoolgirls from the north-eastern town of Chibok in Nigeria's Borno State.
Boko Haram at a glance
- Founded in 2002, initially focused on opposing Western-style education - Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009 to create Islamic state
- Thousands killed, mostly in north-eastern Nigeria - has also attacked police and UN headquarters in capital, Abuja
- Has abducted hundreds, including at least 200 schoolgirls
- Controls several north-eastern towns
- Has launched attacks on Cameroon