IT will be a “bumpy road” to the nuclear negotiations with North Korea later this month, US defence secretary Jim Mattis has warned.

He told his South Korean and Japanese counterparts they must maintain a strong defensive stance so the diplomats can negotiate from a position of strength.

Mattis was speaking at the start of a meeting with South Korean defence minister Song Young-moo and Japanese defence minister Itsunori Onodera on the final day of the Shangri-La Dialogue security conference. He said allies must remain vigilant.

“We can anticipate, at best, a bumpy road to the negotiations,” Mattis said.

“In this moment we are steadfastly committed to strengthening even further our defence cooperation as the best means for preserving the peace.”

Plans are moving forward for a nuclear weapons summit between Donald Trump and Kim Jong Un on June 12 in Singapore.

Mattis repeated the US position that North Korea will only receive relief from UN national security sanctions when it demonstrates “verifiable and irreversible steps” to denuclearisation.

Song said: “Of course, given North Korea’s past, we must be cautious in approaching this.”

He added that some of North Korea’s recent measures “give us reasons to be positive and one can be cautiously optimistic as we move forward”.