Schiff says intelligence agency is withholding Ukraine documents from Congress

"I'll say something even more concerning to me, and that is the intelligence community is beginning to withhold documents from Congress on the issue of Ukraine. They appear to be succumbing to pressure from the administration," Schiff, the House Intelligence Chairman, told ABC.

"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," Schiff continued, referring to Trump's upcoming trial. "That is deeply concerning."

The California Democrat said there "are signs that the CIA may be on the same tragic course." Schiff, however, did not elaborate on those "signs," or specify during the aired interview what documents are being withheld.

The National Security Agency has not responded to CNN's request for comment.

Amanda Schoch, the assistant director of National Intelligence for Strategic Communications, said in response to Schiff's remarks, "The Intelligence Community is committed to providing Congress with the information and intelligence it needs to carry out its critical oversight role. The IC is working in good faith with (House Permanent Select Committee on Intelligence) to respond to requests on a broad range of topics and will continue to do so."

On Tuesday, Trump's Senate trial kicks off in earnest. Schiff, whose committee led the House impeachment inquiry into the President's dealing with Ukraine, is in charge of the six other impeachment managers who will present the Democratic Party's case against Trump to the Senate.

Democrats have pushed in recent weeks to include additional testimony and documents in the trial, an effort that has been stymied by Senate Republicans.

Among the additional witnesses Democrats want to hear from are former national security adviser John Bolton and acting White House chief of staff Mick Mulvaney. They've also pointed to recently released documents provided to the House by indicted Rudy Giuliani associate Lev Parnas as evidence that the trial should include witnesses and additional documents.