The US Special Representative for North Korea has stressed that Pyongyang must clearly state its intention to give up all weapons of mass destruction before trying to improve relations with the United States.

Stephen Biegun made his comments during a lecture on nuclear policy in Washington on Monday. It was his first public speech since the second US-North Korea summit ended last month without an agreement.

Biegun said "diplomacy was still very much alive." Although he did not mention any schedule for future talks, he said US negotiators remain closely engaged with their counterparts in North Korea.

Biegun added that the US wants to build confidence with the North, formally end the Korean War and seek the return of remains of US troops who went missing during the conflict.

But he said all these "require North Korea to be fully committed to the elimination of its weapons of mass destruction."

Biegun also referred to a report by US researchers that North Korea's Sohae rocket launch facility is back in normal operational status, according to analysis of satellite imagery.

The US envoy said the US government is taking the report "very seriously" and is keeping watch. He added: "Right now, I don't know what message they're trying to send."

NHK