By Abdi Sheikh and Feisal Omar

MOGADISHU (Reuters) - Islamist militants al Shabaab seized control of two towns over two days in southern Somalia and attacked two African Union (AU) convoys in the same region, a spokesman for the group and a local official said on Saturday.

Both captured towns were on routes toward Indian Ocean ports south of the capital Mogadishu and came after al Shabaab fighters attacked an AU base in the same area on Sept. 1.

Last year, African Union peacekeepers known as AMISOM drove al Shabaab back in the south and seized the port of Barawe in October, a coastal town used by the Islamist militants to bring in arms and fighters from abroad for years.

Al Shabaab lost control of the main southern port of Kismayu and the port of Marka in 2012, so the loss of Barawe was a setback, leaving the group with no access to the sea in the south of Somalia.

On Saturday, al Shabaab said it had seized two small towns in the Lower Shabelle region: El Saliindi, 65 km (40 miles) south of Mogadishu en route to Marka, and Kuntuwarey, on the road between the capital and Barawe.

The al Qaeda-affiliated group, which is seeking to overthrow the Western-backed government and impose its harsh version of Islamic law, attacks the AU-led peacekeeping force and Somali officials regularly.

CONVOYS

Ali Nur, the acting governor of Lower Shabelle, confirmed the towns had been captured.

"It is sad to say al Shabaab has taken El Saliindi. AU forces withdrew and al Shabaab now controls it," said Nur, adding that Kuntuwarey was seized on Friday.

The AU peacekeeping force, known as AMISOM, could not immediately be reached for comment.

Also on Saturday, the militants said they attacked an AU convoy outside Marka, killing several soldiers. But Nur said the death toll had not been determined.

"A roadside bomb targeted an AU vehicle as the AU convoy passed in the outskirts of Marka today. We do not know details about casualties," said Nur.

Al Shabaab said it hit a second AU convoy with a suicide car bomb near Janale, the site of the attack on the AU base.

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"Many soldiers died and several AU vehicles are burning," Sheikh Abdiasis Abu Musab, al Shabaab's military spokesman, told Reuters.

Nur said he was aware of the bombing. He said while AMISOM forces had not been hit, one civilian was killed.

Al Shabaab often exaggerates the success of its attacks and officials play down losses.

On Sept. 1, al Shabaab stormed an AU base in Janale, about 90 km (55 miles) south of Mogadishu, killing at least 12 Ugandan soldiers. Al Shabaab claimed it had killed 70 people in the assault, which came roughly a year after its leader Ahmed Abdi Godane was killed in a U.S. air strike.

(Writing by Edith Honan; editing by David Clarke)