U.S. security team attacked in Yemen, bomb injures 22 By Mohammed Mukhashaf ADEN (Reuters) - A gunman opened fire on a U.S. security team as it trained Yemeni soldiers in the south of the country, the Pentagon and a security official said on Friday, both denying reports from an Islamist group that a CIA officer was killed in the assault. In the north of the country, a bomb blast hit an anti-U.S. protest, injuring at least 22 people, a rebel group that controls much of the region said. Fighting with northern "Houthi" rebels and southern militants are among a list of challenges facing Yemen's new president, Abd-Rabbu Mansour Hadi, a man Washington was hoping could end a year of turmoil in the country, which is near key oil shipping routes. Ansar al-Sharia, a Yemen-based Islamist group linked to al Qaeda, said it had attacked and killed a U.S. officer in the south on Thursday. "The mujihadeen (holy warriors) killed a CIA officer on Thursday while he was in Aden province, after tracking him and determining he was cooperating with the Sanaa government," it said in a text message sent to journalists in Yemen. A Yemeni security official in Aden, speaking on condition of anonymity, said a gunman fired on a U.S. security team on Thursday, but the shots had hit their armored vehicle without injuring anyone. In Washington, a Pentagon spokesman confirmed that a gunman fired several shots at an armored vehicle carrying a U.S. security team in Yemen and that no one was injured. Fears that political turmoil in Yemen would embolden al Qaeda's wing in the country led Washington and Gulf Arab states to broker the exit of Hadi's predecessor Ali Abdullah Saleh after a year of mass protests against his rule. Saleh's exit made him the fourth Arab leader removed from power since mass uprisings and war swept east from Tunisia in late 2010, rattling U.S. client states and foes alike. Hadi, who won a one-candidate election to replace Saleh last week, has promised to lead Yemen to new elections, create a new constitution and a reform the military within two years. He inherits multiple conflicts including clashes with secessionists in the south and a long-simmering insurgency in the north. Islamist militants took control of swathes of territory in southern Yemen during the struggle over Saleh's fate, and hold the city of Zinjibar in southern Abyan province. A bomb exploded during a protest against U.S. involvement in Yemen on Friday, injuring 22 people in the north of the country, the leader of Shi'ite Muslim rebels that control the area said. In a statement, the leader of the Houthi movement - Shi'ite rebels that Yemen's military tried to crush in campaigns in 2004-2009 - said the U.S. ambassador in Sanaa was personally responsible for the bombing in Saada, near the Saudi border. "The one behind this attack is the U.S. ambassador and his agents," Abdel Malek al-Houthi said. It targets our rejection of foreign interference from the ambassador such as interfering in the structure of the army ... and his satanic efforts to change its creed. The region has seen fighting in recent months between the Houthis and Sunni Muslims espousing puritanical Salafi doctrines influential in Saudi Arabia that cast Shi'ites as heretics. Souther secessionists have rejected the transition deal that put Hadi in power, saying they want northing to do with a process involving a united Yemen. In the capital Sanaa, thousands of protesters gathered near Hadi's residence demanding the military be shaken up to exclude Saleh's relatives and loyalists, witnesses said. Additional reporting by Mohammed Ghobari and Tom Finn in Sanaa; Writing by Joseph Logan; Editing by Andrew Heavens