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WASHINGTON — Democratic lawmakers leading key House national security committees accused President Donald Trump Thursday of keeping Congress in the dark about his administration's talks with North Korea and the state of the regime's nuclear weapons program.

"There is no legitimate reason for having failed to provide regular, senior-level briefings to the relevant committees of jurisdiction on a matter of such significance to our national security," Reps. Eliot Engel, Adam Smith and Adam Schiff, chairmen of the House foreign affairs, intelligence and armed services committees, wrote in a letter to the president.

The lawmakers said "our ability to conduct oversight of U.S. policy toward North Korea on behalf of the American people has been inappropriately curtailed by your administration's unwillingness to share information with Congress."

Trump plans to head to the Vietnamese capital of Hanoi next week for a second meeting with North Korean leader Kim Jong Un. But the Democratic lawmakers said Congress has yet to have been briefed by Secretary of State Mike Pompeo on what transpired at the last summit between the two heads of state in Singapore in June last year.

Democrats in the Senate have been frustrated for some time over the frequency of briefings from the administration, a staffer told NBC News.

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But the Republican chairman of the Senate Foreign Relations Committee, Sen. Jim Risch of Idaho, disagreed.

The senator "communicates with the State Department, the National Security Council, and the White House regularly on this issue, even as recently as this week," spokesperson Kaylin Minton said in an email. "So needless to say, he has a different view."

The White House and State Department did not respond to requests for comment.

In their letter to the president, the Democrats cited recent comments from top intelligence officials and military leaders that North Korea was unlikely to completely abandon its nuclear weapons arsenal, saying it contradicted the president's portrayal of the negotiations.

North Korean leader Kim Jong-Un inspecting a launching drill of the medium-and-long range strategic ballistic rocket Hwasong-12. KCNA / AFP - Getty Images file

"We are perplexed and troubled by the growing disconnect between the Intelligence Community's assessment and your administration's statements about Kim Jong Un's actions, commitments, and intentions," the Democrats wrote.

The lawmakers renewed a demand to provide members of Congress and staff access to information on North Korea's nuclear and conventional weapons program. The Democrats said Congress has been cut off from intelligence on those programs and that it had undercut lawmakers' ability to assess the threat posed by the regime.

The Democratic chairs also said the administration has failed to abide by the defense authorization law that required the secretary of defense to deliver a report to Congress by October 2018 on North Korea's nuclear program, which was supposed to serve as a baseline for progress in any talks with Pyongyang. But the administration has still not delivered the report.

At the summit in Hanoi, Trump will try to persuade North Korea to agree to concrete steps to abandon its nuclear weapons program, but the two sides have so far failed to agree on a definition of what denuclearization should entail.

"I don't know if North Korea has made the choice yet to denuclearize, but the reason why we're engaged in this is because we believe there's a possibility that North Korea can make the choice to fully denuclearize," a senior administration official, who was not authorized to speak on the record, told reporters on Thursday.

"And that's why the President has assigned such a priority to engaging with them."

Trump has said that North Korea's decision not to carry out missile and nuclear testing in recent months — and his rapport with Kim — offers proof that his approach is making diplomatic progress.

North Korea, meanwhile, is pushing for lifting international economic sanctions, which it blames in part for contributing to food shortages in the isolated state. North Korea's ambassador to the United Nations, Kim Song, appealed for food aid in a recent memo to the UN obtained by NBC News. The senior North Korean official said his country is facing dwindling food supplies and has been forced to reduce food rations for its people, according to the memo.