A British Muslim convert was among 11 jihadi militants killed by the Kenyan army in a firefight with Somali extremist group al-Shabaab, officials said.

Kenya Defence Forces said they believed Thomas Evans, from High Wycombe, Buckinghamshire, died in clashes at an army camp in Lamu County, northern Kenya.

Colonel David Obonyo told Reuters that data and pictures of Evans, who went by the nom de guerre Abdul Hakim, led them to believe the British national was among the victims.

"But we cannot be certain becomes sometimes looks can be deceiving. The necessary forensic investigations are being done including a DNA test," he told the news agency.

Evans, 24, left his home for Somalia in 2011, about one year after converting to Islam, according to his mother Sally Evans.

Last year she told The Sunday Times how her son's character dramatically changed as he became radicalised and left the local mosque to attend an Islamic prayer centre "run from a room behind a shop".

He stopped listening to his favourite hip-hop music and watching television, grew a beard and unsuccessfully tried to convert his mother and brother.

"He would say, 'I want you to convert to Islam otherwise you will burn in hell' and 'I want us to meet in paradise so I want you to convert' and 'If you converted, you'd understand what I was doing'," she told the newspaper.

Evans first tried to travel to Kenya to join al-Shabaab in 2011 but was stopped by anti-terrorism police at Heathrow airport. He was successful a few months later as he travelled to the African country via Egypt.

His mother said every time she received a call or message from her son she relayed the information to British police as she preferred to see him behind bars rather than killing innocent people.

"And if he took other lives, how do you live with this? If he was a suicide bomber, how would I live with that?" she told the Sunday Times.

According to Kenyan authorities he was killed as he tried to storm an army camp. Two soldiers and 11 Islamist militants, including al-Shabaab regional commander Luqman Osman Issa, lost their lives in the dawn attack.

"This is a big victory for us and also a major setback for al-Shabaab because now there is no mainstream commander in the Lamu area," Obonyo told Reuters.

Dozens of Westerners, including Britons, are thought to have travelled to Somalia to fight with al-Shabaab, an al-Qaeda affiliate, in recent years.

Among the most famous examples are British native Samantha Lewthwaite, 29, the widow of one of the 7/7 London bombers, and Omar Hammami, 28, an Alabama native who came to international prominence when he posted a series of YouTube videos in which he rapped about jihad.

The group has been leading an Islamist insurgency aiming to impose a strict interpretation of Sharia law in Somalia since its formation in 2006.

Initially the group exploited lawlessness created by more than two decades of almost interrupted conflict in Somalia to seize control of Mogadishu and large swathes of land, mostly in the southern provinces bordering with Kenya.

Its success triggered a regional military response, with Kenya taking a leading role in an African Union (AU) offensive that eventually forced the Islamists out of most towns and cities.

In retaliation al-Shabaab's militants have sporadically crossed into Kenya in recent years to carry out attacks.

In April its gunmen killed nearly 150 people as they stormed the campus of Garissa University, about 150km inland from the Somali border.

In September 2013 the terror cell targeted the upscale Westgate shopping mall in Nairobi, triggering a three-day siege that resulted in the deaths of 68 people.