"Attacks on the FBI and lying about its work will do lasting damage to our country. How will Republicans explain this to their grandchildren?" tweeted Comey, a former Republican who now identifies as an independent.

At issue are reports that a longtime US government informant approached members of Trump's campaign in a possible bid to glean intelligence on Russian efforts to sway the election.

Trump and his GOP supporters in Congress are now demanding information on the outside informant, claiming it as proof that the Obama administration was trying to spy on his Republican campaign for political reasons.

The White House has negotiated rare access to classified documents for Trump's congressional allies in a briefing expected Thursday.

"Look how things have turned around on the Criminal Deep State," Trump tweeted on Thursday. "They go after Phony Collusion with Russia, a made up Scam, and end up getting caught in a major SPY scandal the likes of which this country may never have seen before!"

The President's comments came a day after he increased pressure on the Justice Department, declining to say whether he has confidence in Deputy Attorney-General Rod Rosenstein. Over the weekend, Trump demanded that the Justice Department investigate the FBI and Justice Department's handling of the Russia probe.

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The Justice Department agreed by expanding an open, internal investigation to determine whether there was any politically-motivated surveillance. And the White House said that Trump Chief of Staff John Kelly would organise the meeting with House legislators to review the documents, although he and other White House staffers would not be present.

FBI director Christopher Wray, national intelligence director Dan Coats and Justice Department official Edward O'Callaghan will meet with House Intelligence Committee chairman Devin Nunes and House Oversight and Government Reform chairman Trey Gowdy.

White House press secretary Sarah Sanders said no Democrats were invited because they had not requested the information, despite calls from legislators for the briefing to be bipartisan.

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The top Democrat on the House intelligence panel, California's Adam Schiff, said the briefing should have been done through the bipartisan "Gang of 8", which includes Republican and Democratic leaders and the top Republicans and Democrats on the House and Senate intelligence panels. That group regularly receives classified briefings.

The New York Times was the first to report that the FBI had an informant who met several times with Trump campaign officials who had suspicious contacts linked to Russia.

The Justice Department's internal probe began in March at the request of Attorney-General Jeff Sessions and congressional Republicans. Sessions and the legislators urged Inspector General Michael Horowitz to review whether FBI and Justice Department officials abused their surveillance powers by using information compiled by Christopher Steele, a former British spy, and paid for by Democrats to justify monitoring Carter Page, a former campaign adviser to Trump.

Horowitz said his office will look at those claims as well as communications between Steele and Justice and FBI officials.

AP