"We are in close contact with the Republic of Korea about our unified response to North Korea, including the need to maintain maximum pressure to achieve a denuclearized Korean Peninsula," a State Department official told Yonhap on background.

"The United States and Republic of Korea will work together through the maximum pressure campaign to ensure that north-south progress is accompanied by advances toward denuclearization," the official added. The maximum pressure campaign is a US-led global effort to dismantle North Korea's nuclear program through greater economic and diplomatic sanctions.

Moon told US President Donald Trump Thursday that he plans to send an envoy to Pyongyang to reciprocate the recent visit of North Korean leader Kim Jong-un's sister, Yo-jong. The sister delivered an invitation from her brother to Moon to visit Pyongyang for what would be the third inter-Korean summit.

Moon told Trump in the phone call that his envoy would "confirm details" of issues discussed during Kim Yo-jong's visit to Seoul.

The White House made no mention of Moon's plan in a readout of the call.

"We are willing to engage North Korea to emphasize our position that the complete and verifiable denuclearization of the Korean Peninsula is non-negotiable," the State Department official said. "We are not going to make the same mistakes as previous administrations."

Past negotiations between Washington and Pyongyang have led to agreements to halt the nuclear program in exchange for aid, but they ultimately broke down.(Yonhap)