The top Democrat on the House intelligence committee said Monday that the committee's chairman should recuse himself from any further involvement into the investigation of possible ties between Russian officials and President Donald Trump's campaign.

In a statement, Rep. Adam Schiff, D-Calif., said he believed "the public cannot have the necessary confidence that matters involving the President’s campaign or transition team can be objectively investigated or overseen by [Rep. Devin Nunes, R-Calif.]."

Nunes responded in an interview with Fox News' "The O'Reilly Factor" late Monday, telling host Bill O'Reilly, "I’m sure the Democrats do want me to quit, because they know that I’m quite effective at getting to the bottom of things."

Schiff's statement was made hours after Nunes' spokesman revealed that the congressman met on the White House grounds with the source of the claim that communications involving President Donald Trump's associates were caught up in "incidental" surveillance.

"The chairman is extremely concerned by the possible improper unmasking of names of U.S. citizens, and he began looking into this issue even before President Trump tweeted his assertion that Trump Tower had been wiretapped," Nunes spokesman Jack Langer said.

The meeting occurred before Nunes disclosed at a news conference last week that U.S. spy agencies may have inadvertently captured Trump and his associates in routine targeting of foreigners' communications. Trump quickly seized on the statements as at least partial vindication for his assertion that President Barack Obama tapped his phones at Trump Tower — though Nunes, Schiff and FBI Director James Comey have said there is no such evidence.

"Chairman Nunes met with his source at the White House grounds in order to have proximity to a secure location where he could view the information provided by the source," Langer said.

Previously, Nunes would not say where he met his secret source. In an interview with Bloomberg View on Monday, Nunes said the source was not a White House staffer and was an intelligence official.

In addition to the White House itself, the grounds of the executive mansion include an adjacent building with offices for National Security Council and other executive branch employees.

"Because of classification rules, the source could not simply put the documents in a backpack and walk them over to the House Intelligence committee space," Langer added. "The White House grounds was the best location to safeguard the proper chain of custody and classification of these documents, so the chairman could view them in a legal way."

It is unclear exactly what documents Nunes reviewed. White House spokesman Sean Spicer would not comment on whether White House officials were involved with Nunes.

"I'm not going to get into who he met with or why he met with them," Spicer said.

The bizarre disclosure about the intelligence reports brought criticism from Democrats, especially those who sit on his committee and are working with him on an investigation into Russia's interference in the 2016 election campaign. That investigation is also looking into possible ties between Trump associates and the Kremlin.

Nunes has repeatedly said the intelligence reports were not related to Russia, which could suggest that Trump associates were in touch with other foreign targets of U.S. intelligence surveillance in November, December or January.

The House investigation into Russia's interference in the 2016 presidential race has been plagued with partisan divisions under Nunes' leadership.

The chairman did not tell Schiff about the meeting at the White House complex. It is highly unusual for a committee chairman and ranking member not to coordinate meetings related to an investigation.

"'I think the chairman has to make a decision whether to act as a surrogate of the White House — as he did during the campaign and the transition — or to lead an independent and credible investigation," Schiff said Sunday on CBS' "Face the Nation."

The disclosure renewed calls for an independent committee to investigate the Russia ties.

Indeed, Senate Minority Leader Chuck Schumer, D-N.Y., called on House Speaker Paul Ryan, R-Wis., to replace Nunes as chairman of the intelligence committee.

"He has not been operating like someone who is interested in getting to the unvarnished truth. His actions look like those of someone who is interested in protecting the president and his party," Schumer said.

House Democratic Leader Nancy Pelosi, D-Calif., said, "Chairman Nunes' discredited behavior has tarnished that office," and said Ryan should insist that Nunes "at least recuse himself" from the Russia probe.

AshLee Strong, a spokeswoman for Ryan, said Monday the speaker has "full confidence that Chairman Nunes is conducting a thorough, fair and credible investigation."

Nunes and Schiff have asked the FBI, CIA and National Security Agency for the names of officials who were cited in intelligence reports. The committee has said it is getting some of what it requested, but has not received everything.

The Associated Press contributed to this report.