Somalia's Al Shabaab has declared war on Islamic State in Somalia in an address carried by radio station Radio Andalus on Friday.

"We have given them a chance to change, but they have continued their wrongheadedness," spokesperson Ali Rage said.

"Our senior command has ordered our fighters to attack and eliminate the 'disease' of IS," Rage added, as he expanded on the offensive code named Disease Eradication.

The spokesperson also accused the movement of "spoiling the ongoing jihad in Somalia".

Al Shabaab is a militant Islamic movement formed in 2006 in response to the Ethiopian invasion of Somalia, following years of unrest after a 1991 coup.

They have led an insurgency against a weak government, and once controlled large swathes of the unstable African country. However, they have been beaten back by an alliance of African Union peacekeepers with international support in the last few years.

Tensions between the group and Islamic State, which found a base in the state of Puntland, have existed ever since IS arrived in 2015.

Al Shabaab, an Al Qaida affiliate, primarily sees itself as a Somali resistance group. A lot of its sway in the region relies on latent levels of nationalism in Somali society.

IS ideology, while remaining in line with extreme Islamic teachings, is much more global in its outlook, which directly threatens Al Shabaab leaders.

Al Shabaab is a state within a state, with a standing army that numbers in the thousands, and power structures that have now stood the test of time. But despite its numerical disadvantage, IS launched attacks on Al Shabaab earlier this month.

On Tuesday this week, IS claimed to have killed 14 Al Shabaab fighters in a video seen by Voice of America.