Two US destroyers capable of tracking and intercepting missiles set sail from South Korea on Monday, days ahead of the expected launch of a North Korean rocket. The USS McCain and the USS Chafee left from the port of Busan, a US military spokesman said. He refused to divulge their destination, but South Korean news reports cited unnamed officials as saying the US ships will monitor North Korea's rocket launch set for April 4-8. North Korea said it will send a communications satellite into orbit, but regional powers suspect the North is using the launch to test its long-range missile technology. The US, South Korea and Japan have warned Pyongyang to drop the plan, saying it would face UN sanctions under a Security Council resolution banning the North from any ballistic activity. Defense Secretary Robert Gates said in an interview broadcast Sunday on Fox television that the US won't try to intercept the North's rocket unless an "aberrant missile" were headed to Hawaii "or something like that." Further heightening tensions with Seoul, North Korean authorities detained a South Korean who works at a joint industrial zone in the communist nation for allegedly denouncing Pyongyang's political system, Seoul's Unification Ministry said Monday.