* Attack on bases follows retaking of Janale town

* Rebels have now seized several towns

* Officials have said gains have little strategic value (Adds attack on Friday by Islamists on two bases)

By Abdi Sheikh and Feisal Omar

MOGADISHU, Sept 18 (Reuters) - The Islamist militant group al Shabaab overran two Somali army bases on Friday and killed at least seven soldiers, both the group and the military said, a day after the group retook a southern town.

The raids on the bases in the town of Yaqbariweyne in the Lower Shabelle region and the retaking of Janale town after African forces quit are the latest gains in an offensive by the group that has helped them seize several towns this month.

The government has dismissed al Shabaab's gains, saying the areas retaken have no strategic value. But the offensive has shown the group can keep up pressure on the Western-backed government.

"We took the two Yaqbariweyne military bases in a fierce fighting this morning," Al Shabaab's military operations spokesman, Sheikh Abdiasis Abu Musab, told Reuters. Thirteen soldiers were killed, he said.

Osman Abdullahi, a military officer, told Reuters seven soldiers were killed and five others wounded.

"The soldiers defended themselves and now they are fighting al Shabaab in the outskirts of the town," he said.

Rebels and officials often cite different casualty figures.

Farah Ali, a resident of Yaqbariweyne, which lies northwest of the capital, Mogadishu, said he had counted six dead soldiers and said the town was now calm.

On Thursday, al Shabaab said it had retaken Janale, about 90 km (55 miles) south of Mogadishu. Ali Nur, the acting governor of Lower Shabelle, confirmed the town's capture.

"This is a great shame and it will be difficult for the residents to trust the Somali government and the AU troops," Nur told Reuters. He said the group now controlled much of Lower Shabelle, a region that extends to the south of Mogadishu.

A military offensive last year by the African AMISOM force and the Somali army pushed al Shabaab out of major strategic centres and into smaller pockets of territory.

But the group, which once ruled much of Somalia, still holds some settlements and rural areas, which can often be used to isolate towns by cutting off supply routes. (Writing by Edmund Blair; Editing by Robert Birsel, Larry King)