The African Union peacekeeping mission in Somalia says it had no part in the arrest of the former No 2 leader of the al-Shabaab extremist group, who has been a leading candidate for a regional presidency.The statement released overnight called for "utmost restraint" after several deaths were reported in the gunfire-fueled uproar around Muhktar Robow's arrest on Thursday in Baidoa.His arrest is seen as a high-profile test of Somalia's treatment of defectors from the al-Qaeda-linked al-Shabaab, Africa's most active extremist group. Somalia's government welcomed the defection last year by al-Shabaab's former spokesperson but not his popular candidacy to lead Southwest state, which took some officials by surprise.Robow was seized by Ethiopian troops accompanied by Somali police, witnesses told The Associated Press. He was flown to the capital, Mogadishu, a Somali intelligence official said. All spoke on condition of anonymity because they were not authorised to speak to reporters or for safety concerns.Ethiopia's military, which contributes troops to the AU mission, has not commented. Robow's arrest could re-ignite old tensions between Somalia and neighbouring Ethiopia despite recent diplomatic breakthroughs in the Horn of Africa sparked by Ethiopia's reformist new prime minister.Somalia's security ministry confirmed Robow's arrest, citing the federal government's earlier ban on his candidacy, which said he had not completed the defection process. The ministry also alleged that Robow had failed to renounce extremist ideology, and accused him of mobilising armed forces to threaten the security of Baidoa.Somali officials have announced that the election for the Southwest presidency will go ahead on Wednesday, even after Robow was arrested. His local supporters in Baidoa have loudly protested.A new joint statement by the United States, more than a dozen countries, the AU mission and the United Nations expresses concern, deploring the violence, urging dialogue and urging all parties to "to respect the integrity of the electoral process."Robow's controversial campaign has further exposed the rift between Somalia's federal government based in Mogadishu and regional governments, who in recent months have effectively severed cooperation with the capital over multiple grievances.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.