The UN Security Council unanimously agreed Wednesday to increase the African Union force in Somalia from 12,000 to 17,731 troops.
A resolution passed by the 15-nation council gave the African force a tougher mandate to attack Al-Shabab Islamist militants and substantially increased international funding for the military operation.
The resolution was prepared by Britain ahead of an international conference in London on boosting support for efforts by Somalia's transitional government to re-establish control in the country.
Somalia has had no effective government for more than two decades and in recent years Al-Shabab rebels, which are linked to Al-Qaeda, and other militant groups have taken an increasing hold on large parts of the country.
The African Union force, AMISOM, has been helping the government to fight back over the past year, however. The force had an upper limit of 12,000 but can now call on extra troops following the new resolution.
Kenyan troops already in Somalia will now come under AMISOM command. Ethiopian troops, which have taken the Al-Shabab stronghold of Baidoa, will not be part of the force, however.
The Security Council ordered AMISOM to move into new parts of Somalia and gave it a direct mandate to go on the offensive against Al-Shabab.
AMISOM was "authorised to take all necessary measures" with Somali security forces "to reduce the threat posed by Al-Shabab and other armed opposition groups in order to establish conditions for effective and legitimate governance across Somalia," said the resolution.
AMISOM is run by the African Union but paid for by the United Nations.
The resolution increased international funding for its logistics. Diplomats said the annual cost would increase from about $250 million a year to about $550 million.
The European Union is the biggest contributor to the AMISOM fund.