British troops will be sent to fight Islamic militants in Somalia, David Cameron has announced.

Up to 70 personnel will join a United Nations peacekeeping contingent in support of African Union troops who are fighting the Islamist extremist group, al-Shabab. 

An additional 300 troops could be deployed to South Sudan to help with engineering, combat training and to take an advisory role.

Since the civil war broke out in South Sudan in 2013, the humanitarian crisis has been worsening. 2.2 million people have been displaced and 4.6 million have severe food insecurities, according to the UN humanitarian office.

"Our commitment to peacekeeping operations will help to alleviate serious humanitarian and security issues in Somalia and South Sudan," said Cameron in a statement.

The Prime Minister, who is due to pledge the support at the UN General Assembly today, said the approach could help tackle the number of migrants coming to Europe.

"Obviously we will want to see all the right force protection arrangements in place but we should be playing a part in this," the PM said.

"The outcome in Somalia, if it's a good outcome, that's good for Britain," he added.

"It means less terrorism, less migration, less piracy.

"It's not committing troops to conflict, it's committing troops to a UN blue-hatted peacekeeping role - as we've done many times in the past, as we will do in the future," he told the BBC.

Al-Shabab, which means 'The Youth' in Arabic, are a radical wing of Somalia's former Union of Islamic Courts.