SEOUL, South Korea — North Korea ignored U.N. resolutions by firing a medium-range ballistic missile into the sea on Friday, Seoul and Washington officials said, days after its leader Kim Jong Un ordered weapons tests linked to its pursuit of a long-range nuclear missile capable of reaching the U.S. mainland. South Korea’s Joint Chiefs of Staff said in a statement that the missile fired from a site north of Pyongyang flew about 500 miles before crashing off the North’s east coast.