Seven people, including two MPs, have been killed in a suicide attack in central Somalia, authorities in the town of Dusa Mareb have said. Eyewitnesses told the BBC the attack targeted a group of about 20 politicians at an outdoor cafe. They had travelled to Dusa Mareb in Galgadud region - which is controlled by a pro-government militia - to promote reconciliation. The Islamist militant group al-Shabab has said it was behind the attack. Al-Shabab is under pressure on a number of military fronts in the south of Somalia - but still mounts frequent attacks and controls much of the country. 'Chewing khat' Al-Sunna Wal Jama'a, the militia which controls much of Galgadud, said the suicide attacker was among the dead. Several other MPs and prominent politicians - including former Security Minister Ahmed Abdi Salam - were also injured in the attack, officials told the BBC. Two civilians and two security guards were also killed. Al-Shabab, which merged with al-Qaeda in February, phoned the BBC Somali Service to say it was behind attack on the tea shop where the politicians were resting after holding meetings with residents on Tuesday morning. MP Dahir Amin Jesow, who was part of the delegation, told the BBC the attacker was a young man who approached the group casually chewing the mild narcotic leaf khat. Once he was among the group, he detonated an explosive device shouting "Allahu Akbar (God is great)". Somali Prime Minster Abdiweli Mohamed Ali condemned the bombing, telling the BBC it was "an act of terrorism". The Horn of Africa nation has been without an effective central government since 1991 and has been racked by fighting ever since - a situation that has allowed piracy and lawlessness to flourish. The UN-backed transitional Somali government only controls the capital, Mogadishu. Backed by African Union troops it pushed al-Shabab militants out of the city last August, although the group said it was only a "tactical withdrawal" and still successfully targets Mogadishu. Last month, AU troops for the first time deployed to Baidoa, a strategic south-western town, after it was wrested from al-Shabab's control by Ethiopian troops. This year the AU force has been boosted from 12,000 troops to nearly 18,000 to incorporate Kenyan troops which entered Somalia last October in pursuit of al-Shabab militants. They accuse the fighters of being behind various kidnappings on Kenyan soil and of destabilising the border region. On Tuesday, the UN, AU and the regional group Igad warned in a joint statement on Tuesday that the current peace initiative could be jeopardised by "potential spoilers". They made reference to a deal signed by several disparate factions in the country earlier this year - a blueprint for a future government, to replace the current interim administration, whose mandate expires in August. "We have come too far and too much is at stake for us to allow the process to backslide at the exact moment Somalia has its best opportunity for peace in decades," the statement said. It said any groups attempting to or obstructing the peace process could face international restrictions and sanctions.