At least 12 people, including three foreigners, have been killed in an assault by militant Islamists on a UN office in the Somali capital, Mogadishu, a government minister says. The gun-battle inside the office lasted for more than an hour, witnesses said. A cameraman outside the UN Development Programme office told the BBC a suicide bomber blew up a car at its entrance and he saw four gunmen enter. The al-Qaeda-linked group al-Shabab said it was behind the attack. Officials says pro-government forces have now secured the compound. Somalia has not had an effective national government for more than 20 years, during which much of the country has been a constant war-zone. But a UN-backed government now controls most of the country's main cities after ousting al-Shabab. The UN Development Programme office is next to the heavily fortified airport in southern Mogadishu. "A suicide bomber blew himself up at the gate of the UNDP and then several armed fighters went in and opened fire," a senior police official told the Reuters news agency. Interior Minister Abdikarim Hussein Guled said at least 12 people had been killed. They included foreign staff of the UN, he said, Reuters reports. The cameraman, who asked not to be named for security reasons, told the BBC Somali Service that he had personally helped three people to a military ambulance. Local residents have fled and some buildings were seriously damaged after the vehicle exploded, he said. 'Moved to bunkers' As the cameraman was talking to the BBC at about 09:30 GMT, gunshots could be heard in the background. He said a number of security experts had joined the African Union (Amisom) and Somali soldiers in tackling the militants. Just after 10:15 GMT al-Shabab said on its Twitter feed: "We've just contacted the Mujahideen inside the #UNDP and they are still fighting some western mercenaries inside the compound." Later it said it had killed some foreigners in the raid. "The situation is under control now... Somali soldiers along with African Union forces stormed the compound and killed the attackers," Somali police official Abdulahi Osman told the AFP news agency. Amisom described the attacked as "cowardly" and Somali Prime Minister Abdi Farah Shirdon Saaid praised the speed of the security forces' response. "Somali and Amisom security forces responded immediately to the situation after the initial explosion and have brought the situation under control. Sadly we must wait to hear the full details and confirmation of any casualties," he said in a statement. "All our thoughts and prayers are with our UN colleagues today. But al-Shabab will not derail the peace process. They will not stop our recovery. Violence will not win." Earlier a UN official told the Associated Press news agency that during the sustained firing all staff were "moved to the bunkers where it's safe". Andrew, an aid agency worker staying at a nearby hotel, witnessed some of the fighting. "When I heard the explosions I went up onto the roof of the Jazeera Hotel," he told the BBC. "There were six mini-explosions. We saw UN troops on the roof of their compound shooting down at the attackers. An RPG-7 hit the hotel, it hit the side wall on the fifth floor." Al-Shabab, which had been in control of parts of Mogadishu for more than two years, withdrew in August 2011 under pressure from pro-government forces, but continues to launch occasional suicide attacks in the city. It has also been pushed out of other cities, but still remains in control of smaller towns and large swathes of the countryside in central and southern Somalia. The improving security situation has prompted the return of Somalis from the diaspora and allowed UN agencies and foreign embassies to return to the country. Some 18,000 AU troops are in the country supporting the government of President Hassan Sheikh Mohamud who was elected by MPs last September. His administration is the first one in more than two decades to be recognised by the US and the International Monetary Fund (IMF).