North Korea withdrew its invitation for a US envoy to visit the country, accusing Washington of a \"grave provocation\" by mobilizing nuclear-capable bombers during recent military drills with South Korea. An unidentified North Korean Foreign Ministry spokesman said the US flew B-52H strategic bombers in the area during the exercises, RT reported. He called it \"the most blatant nuclear blackmail against us\". Bob King, US special envoy for human rights in North Korea, was scheduled to travel to Pyongyang on Friday to seek release of Kenneth Bae, a US citizen sentenced to 15 years’ hard labor.