UN chief Ban Ki-moon has backed an African Union (AU) proposal to send a regional force to fight Boko Haram in Nigeria, as warplanes from Chad carried out air strikes against the armed group.
Support for the initiative, announced on Saturday at an AU summit being held in Ethiopia's capital Addis Ababa, came amid an upsurge in fighting with the group that has also drawn in Nigeria's neighbours.
The Chadian military said three of its soldiers and 123 rebels had been killed in two days of fighting in northern Cameroon.
Chadian planes then bombed the Nigerian town of Gamboru on Saturday, security sources said.
Speaking at the AU summit, Ban said Boko Haram needed to be "addressed with a regional and international co-operation".
"I welcome the decision of the AU and regional countries to establish an MJTF [Multinational Joint Task Force] against Boko Haram," he told reporters.
"They have committed unspeakable brutality. Not a single country, even the regional countries, can handle this alone," he added.
"The United Nations is ready to fully co-operate with the African Union."
Ban nevertheless said that "military means may not be the only solution".
"There should be very careful analysis of the root causes why this kind of terrorism, and extremism, violent extremism, are spreading," he said.
At least 13,000 people have been killed and more than a million forced from their homes since Boko Haram launched its fight in 2009 to establish an Islamic state.
In its most high-profile move, the group carried out the mass abduction of 276 girls from the town of Chibok in April last year.
The uprising has become a regional crisis, with the four directly affected countries - Cameroon, Chad, Niger and Nigeria - agreeing along with Benin late last year to form a joint force of 3,000 troops, although the force remains inoperational due to disagreements between Abuja and its neighbours.
Officials at the AU summit said military experts would discuss the force on February 5-7 in Cameroon's capital Yaounde.
The pan-African bloc would then seek UN Security Council approval in the form of a Chapter Seven resolution authorising the use of force, plus a "trust fund" to pay for it.