YAOUNDE (Reuters) - The United States supports the creation of a West African force of up to 10,000 troops to fight Nigerian Islamist group Boko Haram, a U.S. defense official said on Wednesday.

The 54-nation African Union has approved the force and has asked the United Nations to endorse it urgently, after attacks by the group in northeastern Nigeria and neighboring Chad, Niger and Cameroon as it seeks to carve out an Islamic state.

U.S. Deputy Assistant Secretary of Defense for African Affairs Amanda J. Dory said on a visit to Cameroon that Washington, one of five veto-holding members of the U.N. Security Council, would back a U.N. resolution.

"The U.S. is providing diplomatic support in terms of engagement in the U.N. Security Council for the awaited resolution authorizing the deployment of a Multinational Joint Task Force by the African Union against Boko Haram," she told state radio.

If approved, the new force would receive U.N. funding and would be likely to result in a bigger and better resourced operation than the offensive currently being mounted against the militants by Nigeria and its neighbors.

Boko Haram has killed thousands of people in northeastern Nigeria in its six-year insurgency and last week pledged allegiance to the Islamic State which has created a self-declared caliphate in parts of Iraq and Syria.

However, a perception that Nigeria was failing to deal with the militants alone, and a growing number of cross-border attacks, prompted Nigeria, Chad, Niger, Cameroon to launch their existing operation to try to contain the militants.

Nigeria government spokesman Mike Omeri said on Wednesday that Nigeria and its allies had recovered a total of 36 towns from Boko Haram.

Diplomats said the African Union Peace and Security Council was due to discuss on Thursday the text of a possible resolution that could then be circulated to the 15 U.N. Security Council members.

Chad's U.N. Ambassador Mahamat Cherif has said he hoped the council could vote on a resolution by end-March.

France, which holds the rotating presidency of the Security Council, has been seeking to rally support for the resolution in time for a vote by early April, diplomats said.

The United States has already helped Cameroon's army security equipment to fight Boko Haram and France is increasing its own West African counter-insurgency force to support regional troops fighting Boko Haram.