Islamist Shabaab fighters attacked a Kenyan military base in southern Somalia on Friday in the second assault by the Al-Qaeda linked group this week.

The attack on the Kenya Defence Forces (KDF) base at Kolbiyow, close to the Kenyan border in Somalia's Lower Juba region, began with suicide truck bombers blasting their way into the camp, followed by militants attacking from different directions.

Shabaab claimed in a statement to have overrun the base, captured military vehicles and equipment and to have killed 57 Kenyan soldiers.

"Fighters have taken control of the base and the overall Kolbiyow area after massacring the Kenyan infidels," the statement said.

KDF spokesman Paul Njuguna denied the Shabaab claims and said Kenyan soldiers had fought back, killing many of the Islamists.

Njuguna said Kenyan soldiers "fiercely engaged an Al Shabaab group which had attempted to attack the camp" before dawn.

"KDF soldiers repulsed the terrorists killing scores," he said, but did not give any figure for Kenyan casualties.

Shabaab frequently overstates the death toll from its attacks while Kenya commonly underplays its losses.

- Anarchy and conflict -

In January last year a Kenyan base at El-Adde was attacked and overrun by Shabaab fighters who claimed to have killed over 100 Kenyan soldiers. The government refused to give its own toll.

The Shabaab, which once controlled much of Somalia, is fighting to overthrow the internationally-backed government in Mogadishu.

It launches regular attacks on government, military and civilian targets and has carried out a series of deadly assaults against foreign soldiers deployed in Somalia.

The African Union Mission in Somalia (AMISOM) is a 22,000-strong force comprising soldiers from Burundi, Djibouti, Ethiopia, Kenya and Uganda.

Over the last two years Shabaab has rampaged through bases manned by Burundian soldiers in Lego, Ugandan troops in Janale and the Kenyans in El-Adde, inflicting high casualties and stealing military equipment each time.

The Kolbiyow raid is the second major attack this week in Somalia, coming three days after 28 people were killed when Shabaab bombers and fighters attacked a hotel in the capital.

Somalia is due to hold a presidential vote in early February, signalling the end of a drawn-out electoral process in which a new parliament has also been selected.

Political infighting and ongoing insecurity scuppered plans for a universal vote in 2016, with lawmakers elected by specially selected delegates.

Somalia has not had an effective central government since the 1991 overthrow of president Siad Barre's military regime, which ushered in more than two decades of anarchy and conflict in a country deeply divided along clan lines.