WASHINGTON - The US Justice Department on Monday agreed to expand its investigation into alleged Russia collusion in the 2016 election to include "any irregularities" in FBI tactics involving Trump's presidential campaign, a White House spokeswoman said.

Trump suggested on Friday that the FBI might have planted or recruited an informant in his presidential campaign for political purposes, citing unidentified reports that at least one FBI representative was "implanted".

The agreement came during a meeting that Trump had with Deputy Attorney General Rod Rosenstein and Federal Bureau of Investigation Director Christopher Wray, the spokeswoman, Sarah Sanders, said.

The Justice Department "has asked the inspector general to expand its current investigation to include any irregularities with the Federal Bureau of Investigation's or the Department of Justice's tactics concerning the Trump Campaign," Sanders said in a statement.

Federal investigators are probing whether anyone in the Trump campaign worked with Russia to sway the election to the Republican candidate. Trump has denied any collusion and repeatedly dismissed the investigation as a "witch hunt".

Trump said in a Twitter post on Sunday that he would demand the Justice Department look into whether the FBI "infiltrated or surveilled the Trump Campaign for Political Purposes - and if any such demands or requests were made by people within the Obama Administration!"

Hours later, a spokeswoman said the department asked its inspector general to expand a review of the process for requesting surveillance warrants to include determining whether there was impropriety or political motivation in how the FBI conducted its investigation. The FBI was looking into Trump election campaign ties to Moscow before Special Counsel Robert Mueller took over the probe a year ago.

"If anyone did infiltrate or surveil participants in a presidential campaign for inappropriate purposes, we need to know about it and take appropriate action," Rosenstein said in a statement on Sunday evening.

Democrats said Mueller and his investigation should be protected, and information, such as about any informant, should not be shared with Congress.

Justice Department "regulations protect this type of information from disclosure to Congress for legitimate investigative and privacy reasons," Senator Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, said in a letter to Rosenstein on Monday.

Trump has shown increasing signs of impatience with the investigation led by Mueller as it enters its second year, saying it was politically motivated and had its roots in the administration of Democratic President Barack Obama.

His Republican allies in Congress, led by House Intelligence Committee Chairman Devin Nunes, have pushed the same message.

In March, the Justice Department's inspector general launched a review into allegations by Republican lawmakers that the FBI made serious missteps when it sought a warrant to monitor a former adviser to Trump's 2016 election campaign.

Justice Department Inspector General Michael Horowitz said his review will examine whether the FBI and Justice Department followed proper procedures when they applied for a warrant with the Foreign Intelligence Surveillance Court to secretly conduct surveillance on former adviser Carter Page and his ties to Russia.

Republican US Representative Lee Zeldin said he and 16 other members of Congress will introduce a resolution on Tuesday alleging Justice Department and FBI misconduct involving surveillance in the Trump-Russia probe.

Neither Trump nor his new lawyer, Rudy Giuliani, provided any evidence of government infiltration into Trump's presidential campaign.

The New York Times, citing people familiar with the matter, reported that the FBI sent an informant to talk to two Trump campaign advisers, Page and George Papadopoulos, after the agency received evidence that the two men had suspicious contacts linked to Russia during the campaign.

Papadopoulos pleaded guilty last fall to lying to FBI agents about his contacts with Russia.

REUTERS