House Democrats were visibly frustrated on Thursday after taking one of their first oversight moves as the new majority, charging that Treasury Secretary Steven Mnuchin wasted their time after they summoned him to Capitol Hill for a classified briefing with the Foreign Affairs Committee.

Seven committee chairmen had written to Mnuchin on Tuesday and asked him to explain by the end of the week why the U.S. decided to ease sanctions on three companies linked to Russian oligarch Oleg Deripaska. Mnuchin told lawmakers in the secretive briefing on Thursday that the Trump administration will keep strict U.S. sanctions on Deripaska and any companies he owns.

Deripaska has emerged as a player in Special Counsel Robert Mueller's investigation over his ties to Trump's former campaign manager Paul Manafort. Deripaska, a close ally of Russian President Vladimir Putin, signed a $10 million annual contract with Manafort in 2006 and the two maintained a business relationship until at least 2009.

But Mnuchin's responses apparently left a decidedly negative impression on Democrats.

"One of the worst classified briefings we've received from the Trump administration," House Speaker Nancy Pelosi told reporters afterward. "The secretary barely testified."

Pelosi added that she was totally "unimpressed" with Mnuchin's briefing, saying that he was not fully forthcoming and was "wasting the time of the members of Congress."

Asked if the House may move to block Treasury's actions, Pelosi said, "We'll see."

The Democrats were asking Mnuchin about a December announcement that the U.S. would lift sanctions on the three companies — the aluminum manufacturing giant Rusal, EN+ Group and the Russian power company JSC EuroSibEnergo. EN+ Group is a holding company that owns nearly 50 percent of Rusal. In the letter, the Democrats said the sanctions deal appears to allow Deripaska to keep "significant ownership" of one of the companies.

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Congress has 30 days to block the move to terminate the sanctions, but Democrats have asked for an extension. Treasury announced the move just before the December recess and before the start of the government shutdown, the Democrats said, and they haven't had enough time to review it.

The Treasury Department maintains that Deripaska will remain blacklisted as part of sanctions that targeted tycoons close to the Kremlin, and that the companies have committed to diminish Deripaska's ownership and sever his control.

In a statement released ahead of the briefing, Mnuchin reiterated that Deripaska remains under sanctions, "his property and interests remain blocked, and any companies he controls are also sanctioned."

The three companies were originally targeted because they were owned or majority-controlled by Deripaska. "These entities are undergoing significant restructuring and governance changes that sever Deripaska's control and significantly diminish his ownership," Mnuchin said in the statement. "They have committed to provide Treasury with an unprecedented level of transparency into their dealings to ensure that Deripaska does not reassert control. As a result, these entities will no longer be designated for sanctions."

He said if the companies fail to comply with the terms, they could face the re-imposition of sanctions.

Mnuchin also spoke briefly after the hearing to rebut Pelosi's assertion that he was not forthcoming, saying he "answered all their questions."

"One of the worst classified briefings we've received from the Trump administration." — House Speaker Nancy Pelosi, D-Calif.

House Intelligence Committee Chairman Adam Schiff, D-Calif., said he pressured Mnuchin on concerns that Deripaska and other Kremlin allies will continue to exercise influence over the companies.

"It will be incumbent upon Congress to maintain pressure on the Treasury to explain its reversal of course and why Deripaska or his companies are suddenly deserving of this relief," Schiff said.

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Texas Rep. Lloyd Doggett, a Democratic member of the Ways and Means Committee, said he was also unsatisfied with Mnuchin's answers but saw his attendance as the start of a new era of Democratic oversight of President Trump's administration. Democrats have repeatedly vowed to closely police the White House.

"We are saying to the Trump administration, and to the Russians, we are looking carefully at every transaction you are involved with," Doggett said. "We will exercise our oversight."

Separately, on Wednesday, the House Judiciary Committee chairman, Jerry Nadler, D-N.Y., wrote a letter to Attorney General Matthew Whitaker demanding that he testify about his decision not to recuse himself from Mueller's inquiry into Russian meddling in the 2016 presidential elections. Whitaker had asked to delay the testimony until mid-February, citing the ongoing partial federal government shutdown, but Nadler said that excuse was insufficient.

“I cannot accept your proposal,” Nadler wrote. “We are willing to work with you to identify a mutually identifiable date for your testimony, but we will not allow that date to slip past January 29, 2019 – the day of the President's scheduled address to Congress, when we know you will be in Washington.”

On Thursday, 120 former top Justice Department officials sent a letter of support to Bill Barr, Trump's pick to take on the attorney general job full time. Among the signatories: three former attorneys general, two former FBI directors and two former deputy FBI directors, six former deputy attorneys general and more than 70 former U.S. attorneys.

Fox News' Samuel Chamberlain and The Associated Press contributed to this report.