The blast occurred during a routine military operation near Gofgadud town, some 30 kilometres (18 miles) from the southwestern city of Baidoa, they said.
"Six members of the army including Colonel Mohamed Dheere died on the spot," Hassan Mohamed, a military officer in Baidoa told AFP by telephone.
Several others were wounded, he added, without specifying the number.
Al-Shabaab, an Al-Qaeda affiliated group, claimed responsibility for the attack.
Bombings and other attacks are common in the capital Mogadishu and Baidoa, although few have been recorded in recent months.
Last week, Somalia's government said five of its soldiers died in a battle in which they killed nearly 50 fighters in Ceeldheer north of Mogadishu.
The president of the Southwest state, Abdiaziz Hassan Mohamed, sent his condolences to the families of the soldiers and praised Dheere for his key contribution in the fight against Al-Shabaab.
Dheere "played an important role in ensuring the security of the country," Mohamed said in a statement, adding he will be "remembered" for the operations that defeated the Islamist group.
Al-Shabaab has been waging a deadly insurgency against the fragile central government in Mogadishu for more than 16 years.
Although driven out of the capital by an African Union force in 2011, they still have a strong presence in rural Somalia.
They have carried out repeated attacks against political, security and civilian targets, mostly in Somalia but also in neighbouring countries including Kenya.
Somalia's beleaguered federal government has joined forces with local clan militia against the Islamists, retaking swathes of territory in central Somalia in an operation backed by an AU mission known as ATMIS and US air strikes.
But the offensive has suffered setbacks, with Al-Shabaab earlier this year claiming it had taken multiple locations in the centre of the country.
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