The three envoys last met in Washington on March 7.  They were at the 18th Asia Security Summit, also known as the Shangri-La Dialogue, which brought together defense ministers and top-level officials from major countries. It is quite unusual for the top nuclear negotiators to attend the security forum.  

The officials refused to answer questions from reporters.  

Negotiations between Washington and Pyongyang are at a stalemate following the no-deal summit in February between U.S. President Donald Trump and North Korean leader Kim Jong-un in Hanoi.  

"Lee also plans to meet bilaterally with his Japanese counterpart in the morning and with the U.S. official in the afternoon, though the exact schedule is yet to be fixed," an official well-versed in the matter said.  

The meetings "are to share their assessment of the current situation regarding the North's nuclear issues and to fine-tune their message toward Pyongyang in order to keep the dialogue alive," he added.  

On Thursday, the State Department said in a release that Biegun will visit Singapore from May 31-June 2 to attend the dialogue and to meet the officials "to discuss continued coordination on our goal of the final, fully verified denuclearization of North Korea."  

In his speech on Thursday, Lee called on the U.S. and North Korea to strive further to resume their nuclear talks, stressing the window of opportunity will not remain open indefinitely. (Yonhap)