U.S. Deputy Secretary of State Wendy Sherman has urged North Korea to engage in serious and sustained diplomacy.

The U.S. State Department said on Wednesday that Sherman made the appeal the previous day in a phone conversation with Japanese Vice Foreign Minister Takeo Mori.

The department said the two officials reaffirmed the importance of continued cooperation between their nations in the effort to achieve complete denuclearization of the Korean Peninsula.

Deputy Secretary Sherman reportedly reiterated that the U.S. is prepared to meet with North Korea without preconditions, calling for the North to engage in serious and sustained diplomacy.

The department added that they also agreed that trilateral cooperation between the U.S., Japan and South Korea is of great importance when addressing the North Korea issue and other shared priorities in the Indo-Pacific region.