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.
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.
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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 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.
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.