Somali Islamists threaten more carnage after attack on U.N. base MOGADISHU (Reuters) - Somalia's Islamist al Shabaab rebel group threatened to keep attacking "disbelievers" without respite, a day after launching a deadly assault against the United Nations in the capital Mogadishu. Security was tight on Thursday as Somali army pick-up trucks mounted with heavy machine guns blocked the main road linking the city center with the fortified airport and nearby U.N. base that was targeted. A water canon truck blasted away bloodstains on the street. The al Qaeda-linked militants were driven out of Mogadishu almost two years ago by African peacekeepers and government troops. Wednesday's attack, which killed 22 people including four foreigners, highlighted the fragility of security gains and the insurgents' ability to strike at government-controlled areas. "Our aim is to expel the disbelievers from Muslim lands," Sheikh Abdiasis Abu Musab, al Shabaab's spokesman for military operations, told Reuters. Until that goal is achieved, the disbelievers will never find a safe haven in Mogadishu or in any other Muslim land. Not today, not tomorrow, not as long as a single Muslim is alive. The loss of urban territory and revenue streams in the last two years has weakened al Shabaab as a conventional fighting force, leading it to resort to a guerrilla-style insurgency. Western powers, which have long worried that Somalia could provide a launchpad for militant Islam in east Africa and beyond, fear it could slide back into chaos if local forces cannot cement gains. A Somalia analyst said the government had been distracted by its bid to extend its influence beyond Mogadishu, in particular a row over who controls the strategic southern port of Kismayu, and had not focused enough on keeping its heartland secure. "They've taken their eye off the ball," Abdirashid Hashi of the Mogadishu-based Heritage Institute for Policy Studies told Reuters. They need to re-focus on security in Mogadishu. The militants on Wednesday detonated a car bomb outside the main United Nations compound in Mogadishu before several gunmen breached the perimeter wall. They shot dead four foreigners including one U.N. employee and three contractors. Somali guards pinned the assailants back with volleys of sustained gunfire, preventing the attackers from penetrating deeper into the U.N. premises where some staff sought refuge in a safe zone and others hid in their offices. African Union peacekeepers and government soldiers joined a fierce gunbattle that lasted about 90 minutes before the militant fighters were killed. Somalia's government said some exploded suicide vests. The U.N.'s international staff spent the night at the airport, which is ringed by blast walls and watchtowers and serves as the peacekeepers' main base. The United States condemned the attack, which it said highlighted "the repugnant terrorist tactics al Shabaab continues to use to stand in the way of efforts to ease the suffering of Somali people." At his election in September, Somali President Hassan Sheikh Mohamud said security was "priority number one, two and three." The security forces need rebuilding, but the cash-strapped government faces a struggle to pay and arm recruits, tackle corruption and prevent rebels infiltrating their ranks. Additional reporting by Drazen Jorgic and Richard Lough in Nairobi; Writing by Richard Lough; Editing by Edmund Blair and Mark Trevelyan