外交協議の大使（特使）の場合にも、Ambassador to Tokyo（駐日大使）のごとく、toを使います。toに「駐」の意味があるのでしょうね（cf. adhere to）。
N. Korea envoy had "deep discussions" with Japan on abductions
North Korea's delegate to talks held with Japan this week in Mongolia said Saturday he had "deep discussions" with the Japanese side on the issue of Pyongyang's abduction of Japanese nationals.
Song Il Ho, ambassador for talks to normalize relations with Japan, told Kyodo News in Ulan Bator where the bilateral meeting took place for two days through Friday, "We each gave our opinions about the abduction issue and held deep discussions."
Song also said the overall talks were held "in a more sincere atmosphere than before," explaining that there was a mood geared toward trying to resolve problems.