The U.S. Defense Department is weighing whether a decision to shoot down any suspected inbound North Korean president should go all the way to the president, a top general told Congress on Wednesday. Marine Gen. James Cartwright, commander of the Strategic Command that coordinates U.S. missile defence operations, said the authorization would ideally come from the president and the secretary of defence, but there might not be time enough. "As you can imagine, getting the president, the secretary, the regional combat commander into a conversation and a conference in a three to four-minute time frame is going to be challenging," Cartwright told the Senate Appropriations subcommittee on defence. "So what are the rules that we lay down? "We are working very hard with the secretary to lay down those rules and understand the risks associated with those very quick and timely decisions that are going to have to be made ... when we deal with the North Korean threat," he testified. North Korea, at odds with the United States over its nuclear program, is believed to have the capability to mount a warhead on one of its long-range missiles, Vice Adm. Lowell Jacoby, head of the Defense Intelligence Agency, told Congress last month. Cartwright said the U.S. missile defence system is designed to "characterize" a threat in its first three to four minutes of flight. This could leave as little as three minutes to decide whether to fire ground-based interceptors if the suspected target were Alaska or Hawaii. Since October, the multibillion-dollar missile shield of radars, sensors, interceptors and battle management capabilities has been in a "shakedown" or check-out period similar to that used before a warship enters the operational fleet. The ground-based system's prime contractor is Boeing. Ships equipped with Lockheed Martin Corp. Aegis combat systems also have been patrolling the Sea of Japan to provide long-range surveillance and tracking data to the battle management system. "Is it phone calls that we make? Do we use the command and control system in the displays to inform the national command authority? How are we going to bring them together?" Cartwright asked. Last year, when interceptors were lowered into their silos at Fort Greely, Alaska, and Vandenberg Air Force Base, California, the threat was defined as "two to five missiles coming from North Korea," he testified. Air Force Lt. Gen. Henry Obering, director of the Pentagon's Missile Defense Agency, told the committee that current missile defense fielding activities were a "direct response" to perceived dangers from North Korea and Iran. While two recent aborted interceptor test flights had been "very disappointing," the Missile Defense Agency remained confident in the system's design, its ability to knock out targets and its "inherent operational ability," Obering said.