SEOUL, South Korea—A senior U.S. diplomat on Sunday urged North Korea to refrain from additional missile tests and resume nuclear diplomacy, days after the North fired off its first underwater-launched ballistic missile in two years. Sung Kim, the U.S. envoy on North Korea, spoke after meeting with South Korean officials to discuss North Korea’s recent missile tests while nuclear negotiations between Washington and Pyongyang remain stalled. “We call on the DPRK to cease these provocations and other destabilizing activities, and instead, engage in dialogue,” Kim told reporters, referring to North Korea by its official name, the Democratic People’s Republic of Korea. “We remain ready to meet with the DPRK without preconditions and we have made clear that the United States harbors no hostile intent towards the DPRK,” he said. Last Tuesday, North Korea fired a newly developed ballistic missile from a submarine in its fifth round of weapons tests in …