North Korean leader Kim Jong-un is "taking stock" of US President Donald Trump's surprise decision to accept his invitation for talks on denuclearising the Korean peninsula, according to South Korea's foreign minister.

There has been no public North Korean response to Mr Trump in the nearly two weeks since he stunned the world by agreeing to meet with Kim, a diplomatic gambit with huge stakes for both countries.

"We believe the North Korean leader is now taking stock," South Korean Foreign Minister Kang Kyung-wha said on CBS's "Face the Nation."

"We give them the benefit of the doubt, and the time that he would need to come out with some public messaging."

North Korean officials on Saturday concluded three days of talks with Swedish officials in Stockholm, and further meetings were expected in Finland. Sweden provides consular services for the United States in North Korea.

'Back and forth messages'

But the officials gave no indication where the summit invitation stands.

Mr Kang said a channel of communication has been established, adding, "I'm sure there are back and forth messages."

"But, I think the North Korean leader would also need some time, given the readiness with which President Trump has accepted the invitation to talks."

The invitation was relayed to Mr Trump at an impromptu White House meeting on March 6 by South Korean national security adviser Chung Eui-yong, who previously had met with Mr Kim.

Mr Chung told Mr Trump that Mr Kim had agreed to discuss denuclearisation, and would suspend all missile and nuclear tests until the talks were held.

Mr Trump accepted on the spot, catching his own top advisers off guard.

"I think we were all quite surprised by the readiness of that decision," Mr Kang said.

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She said Mr Kim had "given his word" on his commitment to denuclearisation, a key condition for the unprecedented leader-to-leader talks.

"But the significance of his word is quite, quite weighty in the sense that this is the first time that the words came directly from the North Korean supreme leader himself, and that has never been done before."

She said nothing has been offered to the North Koreans to engage in negotiations, and South Korea made it clear "there will be no reward for dialogue."

Asked if South Korea trusted Mr Kim, Mr Kang said: "It's not a matter of trusting. It's a matter of discussing, and pressing for action. And once you see those actions, then you move forward further."