Deputy Attorney General Rod Rosenstein added that "if anyone did infiltrate or surveil participants in a presidential campaign for inappropriate purposes, we need to know about it and take appropriate action."

In a statement, the Department of Justice said it would ask the Inspector General "to expand the ongoing review of the [Foreign Intelligence Surveillance Act] application process to include determining whether there was any impropriety or political motivation in how the FBI conducted its counterintelligence investigation of persons suspected of involvement with the Russian agents who interfered in the 2016 presidential election."

Ramping up a public war of words between the Oval Office and the law enforcement agency, the president said that he would formally request the Department of Justice probe the FBI's role in investigating his campaign. His remarks come amid reports that the agency had placed a confidential informant somewhere in the Trump campaign's orbit, while he was a still candidate.

President Donald Trump said on Sunday he would demand an investigation into whether the FBI had placed surveillance on his 2016 campaign — and whether such activity was ordered by members of the Obama administration.

Trump's assertions of a mole, either inside or outside his campaign, have yet to be confirmed. On Saturday, The New York Times reported that FBI agents sent an informant to talk to two Trump campaign advisers, after receiving evidence that they had made contacts linked to Russia during the campaign that were deemed suspicious.

On separate occasions, the unnamed operative met with George Papadopoulos and Carter Page, The Times noted. Trump has attempted to cast the source as a spy.

Trump's implication of Barack Obama, also renews a fight the president picked early in his tenure with his predecessor. In March of 2017, Trump boldly asserted that Obama of ordering surveillance of Trump's residence during the 2016 campaign — an accusation which the former president flatly denied. The FBI, then led by James Comey, said there was no information to support the claim.

Last week, Trump reacted to the suggestion of potential FBI spying as being "bigger than Watergate." Meanwhile, The Washington Post reported this week that allies close to the president were working to expose the source.

Trump big deal

Trump and the FBI have been locked in an unprecedented public battle. The president and his allies have long complained about a "deep state" agenda of government officials actively working to undermine his presidency.

The controversy has also stirred angst among other members of the Trump administration, with Rosenstein reportedly telling close confidantes that he was preparing to be fired for his role in the Russia inquiry, NBC News reported last month. The president has repeatedly and pointedly criticized Attorney General Jeff Sessions for recusing himself from the matter.

In an interview with The Post last week, former New York City Mayor Rudolph Giuliani — Trump's personal lawyer — accused law enforcement officials of "covering" for actions committed by the Obama administration.

Meanwhile, Giuliani told The Wall Street Journal in an interview published Sunday that the president shouldn't consent to an interview with Special Counsel Robert Mueller, who is leading the probe into alleged ties between Russia and the Trump campaign, unless the role of a suspected FBI informant was made clear.

Otherwise, the former mayor told the publication, Trump could be "walking into a trap."