UN chief Ban Ki-moon on Saturday backed an African Union proposal to send a regional force to fight Boko Haram in Nigeria, as warplanes from Chad carried out airstrikes against the Islamists.

Support for the initiative, announced at an African Union summit being held in Ethiopia's capital Addis Ababa, comes 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 militants were 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 cooperation."

"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 cooperate 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 told reporters.

At least 13,000 people have been killed and more than a million forced from their homes since Boko Haram launched an insurgency in 2009. The group also 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 will 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 7 resolution authorising the use of force, plus a "Trust Fund" to pay for it.

Diplomats said that while "logistical support" would be forthcoming, financing remained the key obstacle to collective action.

"One challenge of course is to finance this force. The best for us will be within the contributions of the UN, but we haven't explored all the possibilities," said Ismael Chergui, commissioner at the AU's Peace and Security Council.

- Mugabe fallout? -

The AU summit, which wrapped up later Saturday, also saw African leaders name Zimbabwe's President Robert Mugabe to the 54-member bloc's one-year rotating chair.

Mugabe, a former liberation war hero who at age 90 is Africa's oldest president and its third-longest serving leader, is viewed with deep respect by many on the continent. But he is also subject to travel bans from both the United States and European Union in protest at political violence and intimidation of opponents in his country.

"What the West will say or do is not my business," Mugabe told a closing press conference, shrugging off concerns that his appointment would harm the AU's relations with the West.

"For more that 10 years I have been under sanctions, my country has been sanctions. If they want to continue it's up to them but these sanctions are wrong," the president said, but added that "if Europe comes in the spirit to cooperate and not the spirit to control us and control our ways, they will be very welcome."

On Friday, however, Ban told African leaders they cannot afford to ignore the wishes of their citizens and condemned "leaders who refuse to leave office when their terms end" -- saying that "undemocratic constitutional changes and legal loopholes should never be used to cling to power."

Countries including Benin, Burundi, Congo-Brazzaville and Rwanda are all reportedly considering changes to allow their leaders seek a third term.

The summit ended with pledges to continue assistance to Ebola-hit Liberia, Sierra Leone and Guinea, and start work on setting up an African Centre for Disease Control. The heads of state also agreed to work on seeking alternative sources of AU financing -- a thorny issue for the bloc, once heavily bankrolled by toppled Libyan strongman Moamer Kadhafi.

Talks on South Sudan, brokered by the east African regional bloc IGAD, were also scheduled to resume in Addis Ababa -- although mediators said talks scheduled for late Saturday had been delayed until Sunday.