US 'not naive' about North Korea: White House

President Donald Trump immediately touted North Korea's pledge over the weekend to suspend its nuclear and missile tests as a sign of "progress", but on Monday, Sanders emphasised that the US was "not naive in this process" and is "not going to make mistakes from previous administrations", reports CNN.

"We're not going to take the North Koreans at their word... The goal is denuclearisation of the peninsula and we're going to continue the maximum pressure campaign that has been ongoing to North Korea until we see concrete actions taken toward complete and total denuclearisation," she said.

"We're not naïve in this process. We've seen some steps in the right direction but we have a long way to go."

Asked whether the US would continue a phased lifting of the sanctions as a reward for incremental progress, Sanders again said "no sanctions" will be lifted "until we see concrete actions taken by North Korea to denuclearise".

On April 21, North Korean leader Kim Jong-un said that his country's quest for nuclear weapons was "complete" and it "no longer needs" to test its weapons capability - a significant development ahead his historic summits with South Korean President Moon Jae-in scheduled for this Friday and one with Trump slated for sometime in May.

Asked on Monday whether Kim's announcement suspending nuclear tests and missile launches was a positive sign, Defence Secretary James Mattis said there are "reasons for optimism", reports CNN.

"Right now, I think there is a lot of reasons for optimism that the negotiations will be fruitful and we'll see," he said.