Despite three failed raids to free US hostages held by militants, the United States will continue to conduct such operations, officials indicated on Sunday, as President Barack Obama grapples with a spate of kidnappings and killings of American citizens.The latest setback came in a remote area of Yemen early on Saturday, when al-Qaida militants shot American photo journalist Luke Somers and South African teacher Pierre Korkie during a rescue attempt led by US Special Forces.Defense Secretary Chuck Hagel defended the operation and the intelligence that lay behind it, and suggested there would be no wholesale review of US policy."I don't think it's a matter of going back and having a review of our process. Our process is about as thorough as there can be. Is it imperfect? Yes. Is there risk? Yes," Hagel said on a visit to Tactical Base Gamberi in eastern Afghanistan."But we start with the fact that we have an American that's being held hostage and that American's life is in danger and that's where we start. And then we proceed from there," he said.An earlier raid in mid-November to free Somers also was unsuccessful - he wasn't present when US and Yemeni forces arrived - as was a July attempt to rescue American journalist James Foley, held by Islamic State in Syria. Foley was later beheaded.Yet however high-risk, the increasingly frequent rescue attempts seem unlikely to stop now, particularly as Obama holds fast to a policy of refusing to pay ransom for captives.