The United States on Sunday stressed the "urgent need for dialogue and diplomacy" with North Korea after the UN nuclear watchdog reported that Pyongyang appears to have restarted its plutonium-producing nuclear reactor.

The International Atomic Energy Agency (IAEA) said in a report to its board of governors that the five megawatt reactor at the North's Yongbyon nuclear complex has shown indications it has been in operation since July.

"We are aware of this report. We are closely coordinating with our allies and partners on developments regarding the DPRK," a senior US administration official told Yonhap News Agency in an email. DPRK stands for the Democratic People's Republic of Korea, the North's official name.

"This report underscores the urgent need for dialogue and diplomacy so we can achieve the complete denuclearization of the Korean Peninsula. We continue to seek dialogue with the DPRK so we can address this reported activity and the full range of issues related to denuclearization," the official said.

The IAEA report came as South Korea's top nuclear envoy, Noh Kyu-duk, arrived here earlier in the day for talks with his US counterparts, including US Special Representative for DPRK Sung Kim, on ways to resume dialogue with the North.

Noh's US trip also follows Kim's visit to Seoul this month, during which the two nuclear envoys discussed humanitarian assistance for North Korea.

"We are holding discussions with the US to enable humanitarian cooperation with North Korea in various areas," Noh told reporters after arriving in Washington. (Yonhap)