NKorea sanctions stay until nukes go: US

North Korea will have to get rid of its nuclear weapons before the US lifts economic sanctions, US Defence Secretary James Mattis says.

US Secretary of Defence James Mattis says negotiations during the summit with NKorea will be bumpy. (AAP)

There should be "no relief" on sanctions against North Korea until the country achieves full denuclearisation, US Defence Secretary James Mattis says.

"We can anticipate, at best, a bumpy road to the negotiations," Mattis said ahead of a trilateral meeting with the South Korean and Japanese defence ministers on the sideline of the Shangri-La Dialogue in Singapore, South Korea's Yonhap news agency reported on Sunday

"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.

Mattis made the remarks ahead of US President Donald Trump's highly anticipated face-to-face meeting with North Korean leader Kim Jong Un in Singapore on June 12.

The Shangri-La Dialogue is an annual security forum held in Singapore. Mattis was meeting with South Korean Defence Minister Song Yong Moo and Japanese Defence Minister Itsunori Onodera.

"North Korea will receive relief only when it demonstrates verifiable and irreversible steps to denuclearisation," Mattis said.