"I think that they are sincere, but I think they're sincere because of the sanctions with respect to North Korea, including the great help we've been given by China," Trump said in a joint news conference with Swedish Prime Minister Stefan Lofven at the White House. "The sanctions have been very strong and very biting. We don't want that to happen, so we're hoping they're sincere. We'll find out."

South Korean President Moon Jae-in's chief national security adviser, Chung Eui-yong, returned from a trip to Pyongyang earlier in the day and reported on his Monday meeting with North Korean leader Kim Jong-un.

Chung said Kim expressed his willingness to abandon his nuclear pursuit in return for a security guarantee. The reclusive leader also told Chung he is willing to hold "candid" denuclearization talks with the US and would stop all provocations, including nuclear and ballistic missile tests, while talks are under way.

The two Koreas agreed to hold a summit next month.

Trump initially tweeted his reaction, saying "possible progress" was being made in talks with North Korea.

"For the first time in many years, a serious effort is being made by all parties concerned," he tweeted. "The World is watching and waiting! May be false hope, but the US is ready to go hard in either direction!"

Prior to that, Trump tweeted, "We will see what happens!" and linked a news report about the Koreans' meeting.

The US president has said he would hold talks with the North Koreans only if they agree to denuclearize. On Saturday he also said he would not rule out direct talks with Kim, whom he has mocked as "Little Rocket Man."

It was the first meeting between Kim and South Korean officials since he assumed power in December 2011.

Chung is scheduled to travel to Washington later this week to brief US officials on his North Korea trip. He said Pyongyang has asked him to deliver a separate message for Washington. The adviser will be joined by Suh Hoon, chief of the National Intelligence Service, who was a member of the five-member delegation to the North Korean capital.

North Korea's surprise offer came amid a thawing of inter-Korean ties driven by its participation in last month's PyeongChang Winter Olympics. The US has until now largely been skeptical of the North's intentions, citing past agreements that led to a freeze in the nuclear program in exchange for aid but which ultimately broke down.

US Vice President Mike Pence said there will be no change in US policy.

"Whichever direction talks with North Korea go, we will be firm in our resolve," he said in a statement. "The United States and our allies remain committed to applying maximum pressure on the Kim regime to end their nuclear program. All options are on the table and our posture toward the regime will not change until we see credible, verifiable, and concrete steps toward denuclearization."

Pence traveled last month to the PyeongChang Games, where he was scheduled to meet with a high-level North Korean delegation that included Kim's younger sister, Yo-jong. The North Koreans, however, pulled out at the last minute, according to Washington.

US Director of National Intelligence Dan Coats told a congressional hearing Washington would have to learn more before determining whether to agree to talks with North Korea.

"Hope springs eternal, but we need to learn a lot more relative to these talks and we will," he told the Senate Armed Services Committee. "And the I.C. (intelligence community) will continue to do every possible collection and assessment we can relative to the situation that exists in North Korea."

A senior US government official later told reporters to keep some perspective as all past denuclearization agreements with North Korea were broken by the regime.

"North Koreans have earned our skepticism, so we're a bit guarded in our optimism," the official said in a conference call.

He also noted that Pyongyang could still proceed with building weapons of mass destruction without testing them.

"If their plan is to buy time to continue building their arsenal, talks aren't going to get very far at all," he said. "We've seen that movie before ... and we're not about to make the latest sequel with a very bad ending."(Yonhap)