The UN Security Council has passed a resolution paving the way for deployment of extra troops to boost the African peacekeeping force battling armed groups in Somalia.

The 15-member council on Tuesday voted unanimously to allow the deployment of the 4,400-strong force, increasing the existing troops to more than 22,100 and renewing their mandate.

Troops are drawn from Uganda, Burundi, Djibouti, Sierra Leone and Kenya. Ethiopia, which has previously sent troops to Somalia, has said it is considering putting its own separate force in the Horn of Africa nation under the command of AMISOM, or African Union Mission in Somalia.

The extra troops are expected to start arriving in early 2014, diplomats said.

AMISOM backs Somali government forces who have forced al-Shabab fighters out of key cities over the past 18 months.

But the al-Qaeda linked fighters have proved they can still stage major attacks on soft targets such as the Nairobi mall siege in September, which left at least 67 dead.

The Ethiopian government warned on Tuesday that al-Shabab were planning more high-profile attacks.

On top of the troop increase, the Security Council resolution also allowed the deployment of a special guard force to protect the UN mission in Somalia, which has been the target of several suicide bomb attacks.

"As recent attacks show, Shabab continue to pose a threat not just to Somalia, but tot the wider region," said Mark Lyall Grant, UN ambassador for Britain, which has taken a leading role in international efforts to rebuild a functioning state in the lawless East African country.

Lyall Grant told reporters after the vote that the extra troops "should allow AMISOM to go onto the front foot and gain momentum in tackling Shabab."

Source: Agencies