Three extremists were killed on Saturday in a failed attempt by the al-Shabaab group to storm the presidential palace in the Somali capital Mogadishu. The extremists, wearing soldiers uniforms, kicked off the attack with a car bombing near a checkpoint close to the presidential palace after security forces engaged with gunmen. A second car bomb exploded in the same area shortly afterward, police Capt. Mohamed Hussein said. Six people were dead in all including a suicide car bomber, he told The Associated Press, saying the situation had calmed and security in the area was being tightened. "There were skirmishes between security forces and the attackers and then we had a big blast and a huge boom. The blast knocked me down," one witness, Osman Ali, told the AP. The Somalia-based al-Shabaab extremist group, an arm of al-Qaida, often targets high-profile places in the capital. It claimed responsibility for Saturdays attack, saying its fighters were conducting a "major operation" around the palace and nearby SYL Hotel. The attack comes a week after five civilians were killed when two blasts targeted Somalias internal security ministry. Al-Shabaab was blamed for the October truck bombing in Mogadishu that killed more than 500 people in the deadliest attack in the countrys history. The al-Shabaab has been fighting to overthrow the internationally-backed government in Mogadishu for over a decade. Despite losing towns and territory in recent years the group continues to carry out regular bombings and armed raids on government, security and civilian targets in the capital and elsewhere. The United States under the Trump administration has stepped up military efforts in Somalia, including dozens of drone strikes, against al-Shabaab and a small presence of fighters linked to the ISIS group. At least two US military personnel have been killed.