In a unanimously adopted resolution, the Council extended the deployment of the AU Mission in Somalia (AMISOM) – which was created in 2007 – until 31 October 2014.
The maximum size of the African force will now increase from just over 17,700 to 22,126.
The additional troops are expected to start their mission in early 2014, diplomats said.
In addition, the Security Council resolution also allowed the deployment of a special guard force to protect the UN mission in Somalia, which has been hit by several bomb attacks.
AMISOM, made up of troops from Uganda, Burundi, Djibouti, Sierra Leone and Kenya, backs Somali government forces fight against al-Qaeda-linked al-Shabab militants.
The UN-backed Somali troops have managed to drive the militants out of key cities over the past 18 months.
Both, UN Secretary General Ban Ki-moon and the AU had recently recommended enhancements for the Mission, including helicopters and other enablers, so that it can combat the insurgency in the Horn of Africa nation, which is trying to move past decades of fighting and advance peace and development.
Additional Reporting By Agencies