UNITED NATIONS - The addition of 4,000 Ugandan troops to the African Union peacekeeping force in Somalia, a move authorized Wednesday by the U.N. Security Council, should lead to the liberation of the capital of Mogadishu from Islamic militias, Somalia's U.N. envoy said.

"It is a very important step," Ambassador Elmi Duale said in an interview. "They will be able to expand their base, first to liberate Mogadishu and then to expand to other areas in the south."

The Security Council voted, 15-0, to authorize the African Union to add 4,000 troops to the 8,000 that now make up the force known as AMISOM. Uganda has pledged to provide the reinforcements.

The vote followed the announcement that Somalia's two biggest Islamist militias - al-Shabab, which the United States accuses of having links to al-Qaeda, and Hisbul Islam - have combined forces to fight the peacekeeping contingent and the nation's transitional government, which controls only portions of the capital.

"We have been in discussions for a while, but we have joined forces with our brothers," Mohamed Osman Arus, a spokesman for Hisbul Islam, said Monday.

Duale said the efforts of AMISOM and his government would also be boosted by the addition of 1,000 national police who recently completed training in Uganda.

The African peacekeepers have been unable to reestablish government control over the Horn of Africa country. Somalia has been without a functioning central administration since 1991, when ruler Mohamed Siad Barre was ousted.

Al-Shabab began its insurgency in 2007 and has been the strongest threat to the central government since then.