More than 110 Somalis have died in the last week, both from fighting in Mogadishu and brief ICU seizures of Jawhar and other towns.
"We arrived and secured the town," said Abdirahim Isa Adow, a spokesman for the ICU. "It is not in our plans to remain in Jawhar."
Their return comes amid fears that towns like Jawhar may be vulnerable to theft and looting by local clan militias after Somali security forces left.
The ICU fighters have carried out relentless attacks against the Ethiopia-backed government forces since their movement was toppled in early 2007.
Many of their members were forced to flee the country, but those who stayed are said to be younger and more radical.
They oppose all foreign presence in Somalia and have vowed to overthrow the current government which they view as a stooge of outside powers.