No, but the exact regulations are murky. Gerald Ford's 1976 executive order on foreign intelligence activities (issued after the disclosure that the CIA had plans to do in Fidel Castro) explicitly prohibits government employees from engaging in "political assassination." This certainly rules out killing heads of state through covert means. It's unclear, however, who else is off limits. The 2001 Authorization for the Use of Military Force Against Terrorists, a congressional resolution that grants the president the right to use "all necessary and appropriate force" against those who helped commit the Sept. 11, 2001 attacks, arguably licenses the CIA to go after terrorists with impunity.