At least a dozen people, most of them soldiers, were killed in the fighting that began in late afternoon and lasted more than an hour in the town of Elwak.
Defeated Somali troops retreated towards the border some three kilometres away, leaving the jihadists in control, the sources said.
"There was heavy fighting in Elwak this afternoon. Shabaab militants attacked the military base of the Somali national army in the suburbs," said Somali military official Abdukadir Elmi. "We don't have the details yet but there were casualties".
"Shabaab fighters took control of Elwak town after storming the military base," said resident Omar Adan.
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"More than 10 soldiers were killed in the fighting including the commander of the camp".
He said Shabaab fighters entered the town disguised as Kenyan soldiers and riding in military vehicles they had captured from the Kenyan army.
A statement on the jihadists' Andalus radio telegram account claimed control of the town and said dozens of troops had been killed.
The Shabaab, which quit the capital five years ago, continue to launch attacks against government, military, civilian and foreign targets in its fight to overthrow the internationally-backed government.
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