"I think when we talked about the use of red lines in the past with respect to Syria -- the president's red line -- that drawing red lines hasn't really worked in the past," he said.

"He holds his cards close to the vest, and you're not going to see him telegraphing how he's going to respond. ... I don't think that you're going to see the president drawing red lines in the sand, but I think that the action that he took in Syria shows that, when appropriate, this president will take decisive action," he added.

It was in response to a question whether the president has a "red line," a tipping point that could prompt military action when it is crossed.

He referred to the US bombing of a Syrian air base on April 7 to punish the Bashar al-Assad regime for a suspected use of chemical weapons.

The spokesman also said that President Trump had a "really good" meeting with Chinese President Xi Jinping earlier this month in Florida, adding that the results are now "paying off."

He still emphasized the need for the stepped-up cooperation from China and urged it to exert more influence in the region.

"I think you see China playing a much more active role with respect to North Korea both politically and economically, that they can continue to apply pressure to achieve results," he said. "And I think we're going to continue to urge China to exhibit its influence in the region to get better results." (Yonhap)