A British-trained navy officer who joined Islamic State has turned supergrass after being arrested by Kuwaiti authorities, becoming one of the most senior figures to hand over intelligence on the terrorist group.

Kuwait-born Ali Omar Mohammad Alosaimi, 27, was picked up on the Iraq-Syria border on July 4, according to officials.

Alosaimi, who had three years of merchant navy officers' training at South Tyneside College’s Marine School - one of the UK’s most prestigious maritime colleges - left his home in South Shields for Syria in April 2014.

Alosaimi, who has since married a Syrian woman with whom he has a child, is now cooperating with Kuwaiti authorities, who said he has confessed to playing a senior role within Isil.