WASHINGTON — The United States is ready for talks with North Korea when it's clear it will follow through on its commitment to denuclearize, the State Department said Tuesday after a planned visit to Pyongyang by top diplomat Mike Pompeo was shelved.

On Friday, President Donald Trump directed Pompeo to delay his trip, which had been slated for early this week, citing insufficient progress on getting the authoritarian regime to abandon its nuclear weapons, as agreed upon with leader Kim Jong Un in June.

Pompeo's spokeswoman Heather Nauert declined to comment on reports that a tough-worded letter from an aide to Kim had derailed what would have been Pompeo's fourth visit to North Korea this year.

Nauert said the president and his national security team, including Pompeo and national security adviser John Bolton, had judged that "now is not the right time to travel." However, she said diplomatic efforts are "ongoing" though she could not say whether there had been communications between the State Department and North Korea since Friday.

She cited a statement from Pompeo that despite the decision to delay the trip, "America stands ready to engage when it is clear that Chairman Kim stands ready to deliver on the commitments that he made at the Singapore summit with President Trump to completely denuclearize North Korea."

"The world is united behind the need for Chairman Kim to fulfill that commitment," Nauert said.

Trump, who views the reduction in tensions with North Korea this year as a major foreign policy achievement, still voiced respect for Kim on Friday and said he looked forward to seeing him "soon." He laid unspecified blame on China, North Korea's leading trade partner, for the lack of progress. China is currently embroiled in a trade dispute with the U.S.

But Trump's tweet marked a shift in his relentlessly upbeat messaging since the Singapore summit, the first ever held between leaders of the U.S. and North Korea. There is growing skepticism in Washington and beyond that Kim intends to denuclearize without first winning concessions such as sanctions relief or a declaration on formally ending the 1950-53 Korean War.

Defense Secretary Jim Mattis told reporters Tuesday that the U.S. might carry out military exercises with South Korea next spring after having cancelled a major exercise this year as a gesture toward advancing diplomacy aimed at eliminating North Korea's nuclear weapons. Mattis said no decision has been made on when to resume military exercises, but his statements suggested the recent cancellation might not be repeated.