The UN Security Council will on Wednesday vote to increase the African peacekeeping force in Somalia to up to 17,000 troops, diplomats said.

A resolution to be passed by the 15-nation council will also seek a ban on trade in charcoal from Somalia which is used by Shebab militants to raise money and threatens an environmental crisis in the Horn of Africa state.

The Security Council's vote will come one day before an international conference in London on efforts to ramp up support for Somalia's transitional government and to combat Shebab and other militant groups.

The African Union force in Somalia, AMISOM, currently has a UN-mandated upper limit of 12,000 troops. The AU wants this increased to 17,731 troops and police to step up the battle against Shebab, which was last year forced out of most of Mogadishu and has also lost ground in other areas.

The council resolution, obtained by AFP, agrees to the increase and sets the African troops the task of moving into new areas of Somalia "to take all necessary measures" with Somali security forces "to reduce the threat posed by Shebaab and other armed opposition groups."

AMISOM is made up of troops from Burundi, Uganda and Djibouti but is to take in Kenyan soldiers now fighting in Somalia. The Security Council will appeal to other African countries to send troops.

Somalia's transitional administration has a mandate until August to set up a structure for a permanent government and parliament. But the weak administration has struggled to overcome divisions between rival groups to increase its authority.

Another provisional political deal was signed by rival leaders at the weekend and the United Nations last month reopened its office in Mogadishu.

But western nations, which pay most of the cost of AMISOM, believe that the opportunity to make decisive gains against Shebab will close in August if a permanent government is not agreed.

The Security Council resolution stresses that Somalia government and AMISOM forces must move into south and central Somalia "on the basis of clear military objectives integrated into a political strategy."

"AMISOM needs a clear military strategy to step up this campaign and there has to be hope that the government will work," commented one Western diplomat of the new international focus on Somalia.

The Security Council also orders the Somali government to take "necessary measures to prevent the export of charcoal" and for all UN members to halt trade in charcoal from Somalia.

The government already bans charcoal exports from areas it controls. But the trade is thriving in rebel areas and has become a valuable source of revenue for Shebab.

Several ships each week are said to leave the Shebab-controlled port of Kismayo. Most head to Gulf states.

But charcoal burning has caused huge damage to the environment. Deforestation has badly hit livestock herding and increased the impact of devastating droughts.