WASHINGTON — House Republicans are stepping up their defense of the rigor of their probe of Russian meddling in the 2016 election, detailing for the first time the full extent of the Intelligence Committee's year-long inquiry, while Democrats warn the committee's work may be coming to a premature end.

Republican officials with knowledge of the investigation tell NBC News that as of late December the committee's Russia Investigative Task Force had completed 67 transcribed witness interviews, for a total of 164 hours of testimony, while reviewing nearly 300,000 documents.

According to the officials, the committee spent 1,200 hours reviewing 2,000 classified source documents that helped shape the intelligence community's January 2017 assessment that Russia aided Donald Trump's candidacy in an escalation of long-running efforts to undermine the U.S.-led democratic order.

House Intelligence Committee Chairman Devin Nunes, R-Calif., center, stands with Rep. Peter King, R-N.Y., left, and Rep. Ron DeSantis, R-Fla., on Oct. 24. Susan Walsh / AP file

Ten investigators and researchers from the committee's GOP staff participated in a process that included visits to seven other countries to question foreign intelligence services, said the officials. The committee itself has held 11 open and closed hearings on the issue, compiling 5,251 pages of testimony for review.

Committee Democrats, however, take issue with some of the GOP data. They say the majority party's tally of witness interviews doesn't match their own, and suspect the Republicans may have double-counted some public testimony or conducted interviews without Democratic participation.

They also say the volume of material the committee has collected cannot be equated with the quality of its work, and that committee Republicans remain unwilling to follow certain investigatory leads or call other key witnesses.

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"No amount of documents produced can obscure the fact that if they persist in refusing to take these investigative steps, they're not very serious about getting to the truth," Rep. Adam Schiff, the top Democrat on the committee, said in an interview.

Rep. Adam Schiff, D-Calif., ranking member of the House Intelligence Committee, speaks to the media after Donald Trump Jr., was interviewed behind closed doors on Capitol Hill on Dec. 6. Carolyn Kaster / AP file

But Republicans say that the figures demonstrate how the committee is following through on its commitment to fully investigate Russian interference.

"For nearly a year, the House review has been following the facts, and it continues to do so," said AshLee Strong, a spokesman for House Speaker Paul Ryan, R-Wisc. "The investigation will conclude when the Committee has reached a conclusion."

The Senate Intelligence Committee is conducting a parallel investigation, with the key difference that up to this point the committee's professional staff, not the committee members themselves, have presided over most interviews.

The chairman of the Senate panel, Republican Richard Burr of North Carolina, said in October that at that point Senate staff had conducted 100 interviews over 250 hours, reviewing almost 100,000 pages of documents including emails, campaign documents and technical cyber analysis products. Another 25 interviews had been scheduled at that time.

Democrats have increasingly made public their concerns that Republicans are short-circuiting the House probe, from refusing to press witnesses to ask key questions, declining to bring in relevant witnesses, and more recently to scheduling multiple key interviews in the same week, or even day. House Minority Leader Nancy Pelosi, D-Calif., recently urged Ryan not to "cut short valid investigatory threads."

More concerning, said Schiff, is the pressure that House Intel Chairman Devin Nunes, R-Calif., has put on the Justice Department and Federal Bureau of Investigation, which Democrats see as part of an effort to undermine the probe led by former FBI Director Robert Mueller.

"The reality is that the majority is far more interested in trying to discredit the FBI and the Department of Justice than they are in looking in to what Russians did in our elections, and the connections" with Trump campaign officials, he said.

But Republicans say it is Democrats doing the "endless political posturing," and said the House Intelligence Committee has an obligation to issue findings on how Russia sought to interfere in elections as soon as possible to prevent it from happening again.

"They would like to see this investigation go on forever. Whether it concludes next month, next year, or in three years, they'll say it's too soon," Strong said.

The House returns to Washington next week, and Republicans have said they expect to conduct additional witness interviews this month. Former Trump campaign manager Corey Lewandowski and Steven Bannon have been invited to testify, though one senior committee source said Bannon at least had not yet agreed to appear.

Schiff said Democrats have been so frustrated in their attempts to convince Republicans to call key witnesses to appear that they may soon make those specific requests public, "so that the public can see in very graphic terms what the majority has deemed unworthy of investigation."

"I hope that step won't be necessary," he added.