* UK fears threat outside Africa if Mali crisis continues
* African leaders seek U.N. mandate for international force
* EU near decision to send in 250 trainers
By Joe Brock
ABUJA, Nov 21 (Reuters) - An Islamist rebellion in northern Mali could become a springboard for jihadists to threaten interests beyond West Africa, Britain's new envoy to the Sahel region said on Wednesday.
Military experts from Africa, the United Nations and Europe have drafted plans to retake control of northern Mali, which fell to rebels in March after a coup in the capital Bamako created a power vacuum.
"This deep insecurity ... we have to recognise that, unless it is checked and it is not met, then it will have the potential for export," said Stephen O'Brien, Britain's first special envoy to the Sahel, a 600-mile (1,000-km) strip of semi-Arid land just south of the Sahara.
In an interview with Reuters while on a visit to the Nigerian capital Abuja, O'Brien said the Mali crisis was "a universal threat" with "the capability of threatening interests outside the ... region."
African leaders will this month seek a U.N. mandate to send a mainly West African force of some 4,000 to Mali to rebuild its army and then back military operations to retake swathes of the Sahara desert from rebels.
Nigeria, whose home grown Islamist movement Boko Haram has made links with Al Qaeda's north African wing (AQIM) in Mali, would commit 600 troops to any intervention, the country's defence ministry told Reuters on Wednesday.
European Union foreign ministers on Monday approved 250 troops to help train Malian soldiers. But, like the United States, the EU has ruled out a combat role.
O'Brien, appointed in September due to concerns about "the developing terrorist, security and humanitarian situation" in the region, said Britain had not yet made any commitment to aid the intervention in Mali but his visit aimed to negotiate a possible role.
France, Spain, Italy and Belgium have indicated willingness to take part in the Mali mission.
Referring to ongoing talks West African mediators are holding with the secular Tuareg rebel group MNLA and the Islamist Ansar Dine, O'Brien said it would be counterproductive to talk with groups that continued to use violence and "terrorist practices".
However, he added: "(If) any part of Ansar Dine is either not using, or is prepared to renounce, violence and to break any links with AQIM, then that becomes a possibility."
Groups that come to a negotiated deal would be spared from the offensive but al Qaeda-linked MUJWA and AQIM are not being considered for talks, O'Brien said. (Editing by Tim Cocks and Robin Pomeroy)