"Well, they won't do it without our approval," Trump told reporters at the White House. "They do nothing without our approval."

He reaffirmed when asked if he had been contacted, "Yes, they do nothing without our approval."

South Korean President Moon Jae-in has been eager to improve relations with North Korea in order to induce its denuclearization and bring lasting peace to the divided peninsula.

An easing of sanctions would be in line with North Korea's wishes, along with a joint declaration with the US to formally end the 1950-53 Korean War.

A State Department spokesperson said the US and South Korea are committed to "close coordination on our unified response to North Korea."

But the official added, in an email to Yonhap, that "President Trump has been very clear that sanctions relief will follow denuclearization, and the sooner we get to that point the sooner we can lift sanctions."

The Trump administration has credited its pressure campaign, involving tightened sanctions, with bringing North Korea to the negotiating table to discuss the dismantlement of its nuclear weapons program.

Trump and North Korean leader Kim Jong-un sat down in Singapore in June for the first-ever summit between the two countries. Kim committed to work toward "complete" denuclearization of the Korean Peninsula in exchange for security guarantees from the US.

Trump said Tuesday that a second summit will be held after the Nov. 6 US midterm elections in one of three or four possible locations. (Yonhap)