South Korean Foreign Minister Kim Sung-hwan telephoned U.S. Secretary of State John Kerry on Tuesday and agreed "close cooperation" over North Korea's apparent nuclear test, a Seoul diplomat said.

"Minister Kim and Secretary of State Kerry discussed measures over the North's possible nuclear test and agreed to closely work together," the diplomat said on the condition of anonymity.

Kim is in New York to preside over the U.N. Security Council's open debate. (Yonhap News)