Several Somali soldiers were killed overnight Sunday when Shebab fighters attacked a base in the southern Kismayo region, officials and witnesses said Monday.

The attack is the latest in a string of assaults by the Al-Qaeda-affiliated insurgents, who last week stormed a base manned by the African Union forces in Somalia (AMISOM), killing dozens.

Heavy gunfire was reported late Sunday for several hours, with the Shebab briefly taking over the camp near Kismayo and beheading some of those captured before looting supplies and leaving, witnesses said.

"There was heavy fighting that left several of the soldiers dead," said security official Abdirashid Moalim, speaking from Kismayo.

"There were machine guns and rockets, and the fighting ended when the soldiers were overpowered," said Abdi Bakar, a witness, who said he had seen nine bodies.

The Shebab is fighting to overthrow Somalia's Western-backed government which is propped up and protected by the 22,000-strong AMISOM force.

On Friday, the Shebab attacked an AMISOM base in Lego village, 100 kilometres (62 miles) northwest of the capital Mogadishu, manned by around 100 Burundian soldiers.

The Shebab later boasted of having killed 80 soldiers, and carried off the bodies of 60 of them, before setting the base on fire.

In a statement the group said the assault had been recorded and promised "images of the attack will soon be published."

Witnesses reported seeing dozens of bodies, but there has been no formal confirmation of casualties and the toll was not possible to confirm independently.

Under military pressure at home, Shebab is increasingly turning its attention to Kenya where it has launched a string of recent deadly assaults, as well as focusing on guerrilla-style warfare in Somalia.

The Islamic militants commonly step up their attacks during Islam's holy fasting month of Ramadan. On Wednesday six people were killed in a suicide attack in Mogadishu on a diplomatic convoy from the United Arab Emirates.