Nigeria violence: Cameroon boosts anti-Boko Haram border forces
Cameroon has deployed about 1,000 troops to its border with Nigeria to fight the growing threat of the Islamist group, Boko Haram.
A defence ministry spokesman said the troops would carry out reconnaissance missions and return fire if necessary.
The group is also said to have killed 11 soldiers and 13 police officers in the north-eastern state of Yobe, the Nigerian military has said.
Yobe is one of the three north-eastern states under a state of emergency.
Witnesses in the town of Buni Yadi said the attackers had arrived in several vehicles, including an armoured car, and told residents they were not going to kill civilians, only members of the security forces.
They also burnt buildings including the home of a traditional ruler and a police station.
Buni Yadi is a notoriously volatile area, where dozens of teenage school boys were killed in their dormitory last February, says the BBC's Will Ross in Abuja.
The almost daily attacks in north-east Nigeria show the massive task the Nigerian military faces, he adds.
More than 500 people have been killed since Boko Haram hit world headlines by abducting more than 200 schoolgirls from their remote boarding school in Chibok last month.
Earlier on Tuesday, the Nigerian military said it knew where the girls were, but would not attempt a rescue.
Retired US Gen Carter Ham, who was formerly one of the commanders of the US Africa Command, which liaises with African countries on military matters, told the BBC that the Nigerian government must now be considering its options:
"There are difficult options at best. Certainly a military or security force hostage rescue operation brings with it great, great risk, and I think there have been in public debate about Nigeria's capability to perform that, but... if they have an opportunity and see that is the right thing to do, they probably could."
The Nigerian military has rejected accusations that it is hampering efforts to find the kidnapped girls because of poor training and corruption.
Responding to an article in the New York Times, it said in a statement that the author of the article failed "to acknowledge the fact that this same Nigerian military have trained and conducted military exercises and operations alongside the military organisations of other nations in recent times and excelled".
Thousands of people have died since Boko Haram began a violent campaign against the Nigerian government in 2009 and in the subsequent security crackdown.Who are Boko Haram?
- Founded in 2002
- Initially focused on opposing "Western" 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 - also attacked police and UN headquarters in capital, Abuja
- Some three million people affected
- Declared terrorist group by US in 2013