WASHINGTON (WASHINGTON POST) - The chairman of the House Intelligence Committee acknowledged that he had made a secret visit to the White House last week to view intelligence files which he then cited as proof of potentially improper spying activity against President Trump, casting new doubt on the independence of a congressional investigation into Russian election interference.

The admission by Devin Nunes, Republican-California, on Monday (March 27) triggered calls among Democrats for his removal as chairman of the House panel, and bipartisan appeals for an independent probe of Kremlin meddling in the 2016 election and potential connections between Russia and Trump associates.

The development coincided with the disclosure that Trump's son-in-law and close adviser, Jared Kushner, had privately met in December with the chief executive of a Russian bank being targeted by US sanctions, and that Kushner has agreed to discuss such contacts with the Senate Intelligence Committee.

Trump administration officials sought to play down the significance of both developments, describing Kushner's contacts as inconsequential and refusing to answer questions about the Nunes visit.

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

The meeting with a source and his review of intelligence material apparently occurred in a secure space for handling classified files within the Old Executive Office Building adjacent to the White House. Nunes returned to the White House the next day - bypassing colleagues on the House committee - supposedly to brief Trump on what he had learned.

The attempts to keep such matters hidden from public view, however, added to the perception that the Trump administration has failed to be forthcoming about contacts with Russia and is working with allies on Capitol Hill to blunt congressional probes.

The Senate's top Democrat said that House Speaker Paul Ryan, Republican-Wisconsin, should remove Nunes to salvage that chamber's investigation of Moscow influence.

"If Speaker Ryan wants the House to have a credible investigation, he needs to replace Chairman Nunes," Senator Charles Schumer, Democrat-New York, said.

Asked about Nunes' White House visit, Senator Lindsey Graham, Republican-South Carolina., said: "Not good. It's not a confidence builder."

He said "we're rapidly getting" to the point where a select committee or independent commission is needed to conduct the investigation into Russian meddling.

Nunes said in an interview on Monday that no one in the Republican leadership had asked him to step aside, and he defended his actions as part of an attempt to investigate potential misconduct by US spy agencies against Trump associates.

"Everybody is worried by process and they should be worried about what I've actually said about what I've seen," Nunes said, when asked whether it was proper for him to visit the White House under those circumstances.

"Why all the worry about where I saw information? We go to the White House all the time, our job is providing oversight of the executive branch."

Nunes had previously refused to say how or where he had seen classified files he cited in a hastily arranged news conference last week, saying that he had obtained troubling evidence that US spy agencies "incidentally collected information about US citizens involved in the Trump transition."

At a time when the White House was struggling to defend Trump's baseless accusation that he had been wiretapped under orders issued by then-President Barack Obama, the Nunes assertion helped shift public attention and, to some, cast Trump as a victim of espionage abuse.

In reality, Nunes appeared to be referring to legitimate intelligence operations against foreign individuals who were either in contact with Trump associates or mentioned them in conversations that were monitored as part of routine US surveillance. Nunes reiterated onMonday that he has seen no evidence of illegality.

Current and former national security officials described Nunes's trip to the White House complex, apparently late in the evening after he had slipped away from his staff, as highly unusual. Doing so would ordinarily require Nunes and the person he met with to have been cleared in advance and accompanied by an escort - requirements that seemed to undercut White House claims to have no information about the encounter.

"How incredibly irregular," said Matt Olsen, who served in the Obama administration as the head of the National Counterterrorism Centre and the general counsel at the National Security Agency.

"The only explanation you're left with is that this is all being orchestrated by the White House."

Nunes again declined to disclose with whom he met, citing the need "to protect people who bring information to the committee, and I'm going to protect my source." His office said he met the source on the White House grounds.

The House Intelligence Committee is authorised to handle classified information and routinely meets with officials - including whistleblowers - from US spy agencies.

Nunes spokesman Jack Langer said that because of limitations on House computer systems, Nunes could not have used secure facilities at the Capitol to review the files. He added that "the White House grounds was the best location to safeguard the proper chain of custody and classification of these documents."

Nunes has said that the documents include references to Trump advisers and associates, but do not pertain to Russia. In the past few days, former Trump campaign chairman Paul Manafort and former campaign advisers Carter Page and Roger Stone volunteered to make themselves available for interviews with the Senate and House Intelligence committees.

On Monday, officials from the White House and Senate said that Kushner had also offered himself for an interview with the Senate Intelligence Committee, at a date yet to be determined. The development was first reported by the New York Times.

A senior congressional official said Senate Intelligence Committee Chairman Richard Burr, Republican-North Carolina, spoke with the White House counsel "some weeks ago" to warn that the panel would be seeking to speak with administration officials, including Kushner. The White House indicated to the committee over the weekend that Kushner would be willing to participate.

The White House had previously disclosed that Kushner met with Russian Ambassador Sergey Kislyak at Trump Tower in December, a session also attended by former national security adviser Michael Flynn, who was fired for lying about the nature of his contacts with Kislyak.

On Monday, the White House acknowledged a previously undisclosed meeting between Kushner and Sergey Gorkov, chief of Russian government-owned Vnesheconombank. The bank, which handles Russia's pension funds and deals with development activity for the state, including foreign debts and investments, has been under US sanctions since July 2014, in response to Russia's intervention in Ukraine.

The bank also has been tied to Russian intelligence services.

In early 2015, one of the bank's New York-based employees, Evgeny Buryakov, was arrested and accused of being an unregistered spy for Russia's foreign intelligence service, working with two Russian diplomats who were also secretly acting as spies. According to the US government, they collected information about US sanctions against Russia, and American efforts to develop alternative energy resources.

Buryakov pleaded guilty in March 2016 to conspiring to act as an agent of a foreign government, though he never admitted to being an employee of Russia's foreign intelligence service.

Spicer defended Kushner's meetings, saying that he was the "official primary point of contact" with foreign governments and officials during the campaign and transition period.