Addis Ababa - UN Secretary-General Ban Ki-moon on Saturday welcomed an African Union proposal to set up a regional five-nation force of 7 500 troops to fight Nigeria's Boko Haram Islamist militants.
Support for the initiative, announced at an African Union summit being held in Ethiopia's capital Addis Ababa, came hours after the Chadian military said three soldiers and 123 militants were killed in two days of fighting with a Chadian army contingent in northern Cameroon.
"I welcome the decision of the AU and regional countries to establish an MJTF (Multinational Joint Task Force) against Boko Haram," he told reporters on the sidelines of the summit.
"They have committed unspeakable brutality. Those terrorists should be addressed with a regional and international co-operation. Not a single country, even the regional countries, can handle this alone," he said. "The United Nations is ready to fully co-operate with the African Union."
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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 by Boko Haram violence since 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 unoperational due to disagreements between Abuja and its neighbours.
The proposed force was backed by the AU's Peace and Security Council on Friday, and the pan-African body is now seeking UN Security Council approval, plus a "Trust Fund" to pay for it - although diplomats said that while "logistical support" would be forthcoming, it remains unclear whether African nations will secure cash pledges.
Officials at the AU summit said military experts will discuss the force on 5-7 February in Cameroon's capital Yaounde.
The AU summit has also seen African leaders name Zimbabwe's President Robert Mugabe to the 54-member bloc's one-year rotating chair, replacing Mauritania's President Mohamed Ould Abdel Aziz.
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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.
Questioned by reporters on the potential for diplomatic fallout over Mugabe, Ban said the AU "have their own procedures and practices for electing their leadership".
"I respect the will and decision of the African Union. I am ready to co-operate closely with the African Union leadership," he added.
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 said to be considering changes to allow their leaders a third term.
The summit also includes closed-door talks on a string of crises, including Somalia, Mali, Libya, South Sudan and Democratic Republic of Congo.
Talks on South Sudan, brokered by the east African regional bloc IGAD, are also scheduled to resume in Addis Ababa on Saturday.