Somali al-Shabaab militants said British and Turkish special forces had raided a coastal town overnight, killing a rebel fighter, but that a British officer had also been killed and others injured.
A British Defence Ministry spokeswoman said: "We are not aware of any British involvement in this at all."
A Turkish foreign ministry official denied any Turkish part in such an action.
Al-Shabaab said the target of the raid on its stronghold in the small southern coastal town of Barawe was a Chechen commander.
He had been injured and his guard killed.
It was not clear whether the assault was related to the attack on a Kenyan mall two weeks ago.
The al-Qaeda-linked group said it carried out and which killed at least 67.
Nor was there any independent confirmation of what forces were involved.
Both US and French forces have carried out similar raids in the past.
The French army denied involvement and the Pentagon declined to comment.
Al-Shabaab leader Ahmed Godane, also known as Mukhtar Abual-Zubayr, has described the mall attack as retaliation for Kenya's incursion in October 2011 into southern Somalia to crush the insurgents.
It has raised concern in the West over the operations of Shabaab in the region.
Sheikh Abdiasis Abu Musab, spokesman for al- Shabaab's military operations said foreign forces had landed on the beach at Barawe, about 180km south of Mogadishu.
It launched an assault at dawn that drew gunfire from rebel fighters in one of the militia's coastal bases.
He later said the attack was carried out by Britain's SAS unit and Turkish special forces.
The British commander was killed during the raid and four other SAS soldiers were critically wounded.
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.
Neither Turkish nor British forces have any past record of raids in the area.
Barawe is fully controlled by the Islamist militia with almost no government presence.