SINGAPORE, June 3 (Xinhua) -- U.S. Defence Secretary Jim Mattis said there should be "no relief" to U.N.-led sanctions on North Korea until its full denucelarization, citing an expected "bumpy road" to negotiations.

"We can anticipate, at best, a bumpy road to the negotiations," he said at the opening of trilateral talks with his South Korean and Japanese counterparts -- Song Yong-moo and Itsunori Onodera -- held on the sidelines of the Shangri-La Dialouge, a security forum under way in Singapore.

The meeting came as North Korean leader Kim Jong-un and U.S. President Donald Trump plan to hold a summit here on June 12.

As defense chiefs, the secretary said, "We must maintain a strong, collaborative, defensive stance so we enable our diplomats to negotiate from a position of strength at this critical time."

Mattis said the regional powers need to implement all U.N. Security Council sanctions on Pyongyang in place.

"North Korea will receive relief only when it demonstrates verifiable and irreversible steps to denuclarization."