Gen Sir David Richards in al-Qaeda African warning

Malian soldiers transport in a pickup truck a dozen suspected Islamist rebels in February 2013
Image caption Islamist groups have been stepping up their activities in the region

The world should be paying more attention to the sub-Saharan threat from al-Qaeda, a former head of the British armed forces has warned.

General Sir David Richards said Britain needed to learn from what it had done "and failed to do".

He also questioned whether the Nato operation against Libya in 2011 was the right thing to do, suggesting it may have contributed to the spread of arms in the region.

Gen Sir David retired last year.

In the past year Islamist groups have stepped up their activities in several African countries.

In Somalia, African Union troops have been battling with militants al-Shabab - which came to global prominence with its attack on a Kenyan shopping centre earlier this year.

French troops spearheaded the response to Islamist insurgents in Mali.

And in Nigeria a state of emergency has been declared in response to attacks by another group.

"I do worry very much that sub-Saharan Africa is the next front - in many ways it already is," Gen Sir David told the BBC.

"We must learn from what we have done and failed to do in other parts of the world in trying to combat this risk and do it pretty quickly or it could become pretty vicious - as if it wasn't already bad enough."

Image caption As chief of the defence staff, Gen Sir David was the prime minister's main military adviser

The BBC's international development correspondent Mark Doyle said that, despite signs of growth elsewhere in Africa, the militants' actions had caused widespread disruption and held back the region's economies.

Gen Sir David was chief of the defence staff from 2010 until July this year.

But he now says the 2011 Nato-led campaign to remove Colonel Gaddafi in 2011 may have helped the proliferation of arms in Africa.

He said that campaign was a tactical success but that the jury was still out on whether it was strategically wise.

For more on the rise of Islamic militants in Africa listen to the BBC Radio 4 documentary, Lines In The Sand, by our International Development Correspondent Mark Doyle at 20:00 GMT

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