Somali Islamist militants say "Western" forces attacked a house in one of its coastal bases in the town of Barawe under the cover of dark, killing one rebel fighter.
Foreign forces landed on the beach at Barawe, about 180 km (110 miles) south of Mogadishu, and launched an assault that drew gunfire from rebel fighters, Sheikh Abdiasis Abu Musab, spokesman for al Shabaab's military operations, told Reuters.
It was not immediately clear why the Barawe base had been targeted specifically, nor whether the assault was related to the attack on a Kenyan mall two weeks ago, which the al Qaeda-linked group said it carried out and which killed at least 67.
Western navies patrol the sea off Somalia - mired in conflict for more than two decades - and have in the past launched strikes on land from warships.
No foreign military units typically involved in such operations in Somalia have said they were involved in the strike. One Western official in the region declined to comment.