MULTAN, Pakistan (Reuters) - Militants armed with rocket-propelled and hand grenades and a car bomb attacked an office of Pakistan's main security agency in the city of Multan on Tuesday killing nine people, officials said. The bomb attack in the eastern city was the third in Pakistan in two days and underscored the relentless security troubles facing the U.S. ally whose help is vital in efforts to stabilize neighboring Afghanistan. The violence coincides with rising political tension with the Supreme Court hearing challenges to an amnesty order that could heap pressure on President Asif Ali Zardari.