Some 4,000 Ethiopian troops in Somalia on Wednesday formally joined the UN-backed African Union force in the country as it seeks to boost operations against Al-Qaeda-linked Shebab rebels, officials said.
Hardline Shebab insurgents control large parts of rural southern Somalia, and despite having been driven from a string of towns by the African Union Mission in Somalia (AMISOM), guerrilla units stage regular deadly attacks in the capital Mogadishu.
Ethiopian troops crossed into Somalia in November 2011 to battle Shebab militants.
Their inclusion into AMISOM will free up other units to stage a long-awaited offensive on Shebab bases in the far southern regions of Lower and Middle Shabelle, with Kenyan units advancing from the south, and Uganda and Burundi pressing from the north.
After a series of sweeping victories, the force has remained largely still for around a year, hampered by limited troops and air power to advance again.
The Shebab-controlled port of Barawe, one of the last sea access routes for the extremists, is a key target for the force.
"Ethiopian troops will constitute AMISOM's sixth contingent," the force said Wednesday, with soldiers re-hatting to join troops from Burundi, Djibouti, Kenya, Sierra Leone and Uganda.
"The Ethiopian deployment will permit Burundian and Ugandan forces to move into parts of Lower and Middle Shabelle," the mission said in a statement, suggesting preparations for a fresh offensive are gathering pace.
Ethiopia's contribution of three battalions takes the AU force to the 22,000-strong level mandated by the UN Security Council, who last year boosted the authorised force by a third.
Their troops are based mainly in southern border zones, including in the towns of Baidoa and Beledweyne.
"New battalions have been sent and they are on the ground now," said Getachew Reda, spokesman for Ethiopian Prime Minister Hailemariam Desalegn.
"Forces have been deployed into their respective sector within south-central Somalia."
AMISOM, first deployed in 2007, is a unique military operation as it is run by the African Union, but with a UN mandate. Most of its financing comes from the European Union and other international donors.
Ethiopia denied suggestions it was joining AMISOM to win external funding of the military, saying that joining the force would improve coordination in the fight against the Shebab.
"It's not a funding issue, we have been bearing the brunt both financially and materially -- in terms of loss of life -- for the last two years," Getachew told AFP.
"Without harmonising our efforts with AMISOM and other stakeholders... the kind of result we?d like couldn?t really be achieved."
Ethiopia had sent troops into its lawless neighbour in a US-backed invasion in 2006, but the move sparked a bloody uprising and the troops pulled out three years later after failing to restore order.
The Shebab once controlled most of southern and central Somalia but withdrew from fixed positions in Mogadishu two years ago.
But a string of devastating Shebab attacks against foreign and government targets have shattered hopes of a rebirth for the war-ravaged capital and demonstrated that the Islamist outfit's disruptive power is undiminished.
The group also claimed responsibility for last year's deadly attack in neighbouring Kenya, when commandos stormed the upmarket Westgate mall, shooting shoppers and hurling grenades.