National security adviser John Bolton on Monday expressed frustration over the slow pace of North Korea's denuclearization, but said President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE remains open to meeting with leader Kim Jong Un this year.

"The possibility of another meeting between the two presidents obviously exists. But President Trump can't make the North Koreans walk through the door he's holding open," Bolton said in a speech to the Federalist Society, a conservative legal organization.

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Bolton later told reporters the two men could meet "sometime this year," but that such a get-together remains "entirely ... hypothetical."

The top national security official indicated the administration wants to see North Korea speed up efforts to dismantle its nuclear program before a meeting occurs. Bolton said Kim previously told South Korean officials he is capable of getting rid of his weapons within a year.

"We're still waiting for them," he said.

Several media reports, citing intelligence and military officials, say North Korea is pushing ahead with developing nuclear weapons while attempting to hide the efforts from Western nations.

Concerns over the weapons program sparked Trump to scrap a planned meeting last month between Secretary of State Mike Pompeo Michael (Mike) Richard PompeoOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Overnight Defense: House Democrats unveil stopgap spending measure to GOP opposition | Bill includes .6B for new subs | Trump issues Iran sanctions after world shrugs at US action at UN Navalny calls on Russia to return clothes he was wearing when he fell ill MORE and North Korean officials in Pyongyang.

But the president over the weekend praised North Korea for removing ballistic missiles from its annual Foundation Day military parade, calling it "a big and very positive statement."

"Thank you To Chairman Kim. We will both prove everyone wrong! There is nothing like good dialogue from two people that like each other! Much better than before I took office," he tweeted Sunday.

Trump said last week he is expecting a new letter from his North Korean counterpart.