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House Intelligence Committee Chairman Adam Schiff, D-Calif., said Sunday that the National Security Agency is withholding "potentially relevant documents" from Congress about Ukraine just as President Donald Trump's Senate impeachment trial is set to start.

"They appear to be succumbing to pressure from the administration," Schiff said on ABC's "This Week." "The NSA in particular is withholding what are potentially relevant documents to our oversight responsibilities on Ukraine, but also withholding documents potentially relevant that the senators might want to see during the trial.

"That is deeply concerning," Schiff continued. "And there are signs that the CIA may be on the same tragic course. We are counting on the intelligence community not only to speak truth to power but to resist pressure from the administration to withhold information from Congress because the administration fears that they incriminate them."

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Amanda Schoch, the assistant director of national intelligence for strategic communications, said in a statement: "The Intelligence Community is committed to providing Congress with the information and intelligence it needs to carry out its critical oversight role."

Referring to the intelligence community, or IC, she added: "The IC is working in good faith with [the House Intelligence Committee] to respond to requests on a broad range of topics and will continue to do so."

Congressional Democrats have lamented the State and Defense departments' refusal to provide key documents related to Ukraine.

Trump's impeachment trial is set to begin on Tuesday. Democrats and Republicans are still battling over whether additional documents and witnesses will be presented — something that Senate Majority Leader Mitch McConnell, R-Ky., has said will be determined by a vote after initial arguments are presented by both sides.

The first article of impeachment charges the president with abusing his power by pushing for Ukraine to investigate former Vice President Joe Biden, his son, Hunter Biden, and other Democrats while withholding nearly $400 million in military aid to the country, as well as an official White House visit for Ukrainian President Volodymyr Zelenskiy. The second charges him with obstructing Congress' efforts to investigate his conduct.

Schiff on Sunday also addressed a recent Politico report in which multiple sources said the intelligence community is trying to persuade Congress to eliminate public testimony as part of the annual briefing on global security threats in light of Trump's public outburst at his top intelligence officials last year for contradicting his beliefs.

Schiff issued invitations to intelligence agency leaders Thursday to testify publicly and behind closed doors about the global threats.

"Well, you know, unfortunately, I think those reports are all too accurate," Schiff said. "The intelligence community is reluctant to have an open hearing, something that we had done every year prior to the Trump administration, because they're worried about angering the president."