US president Donald Trump has said he will “demand” that the Justice Department opens an investigation into whether the FBI infiltrated his presidential campaign.

The order came hours before his legal team said the special counsel indicated its investigation into the president could be concluded by September.

Mr Trump tweeted: “I hereby demand, and will do so officially tomorrow, that the Department of Justice look into whether or not the FBI/DOJ infiltrated or surveilled the Trump Campaign for Political Purposes – and if any such demands or requests were made by people within the Obama Administration!”

Mr Trump’s pressure on the Justice Department — it asked its watchdog later on Sunday to expand an existing probe of FBI actions — reached a new intensity with the demand, and came amid a White House strategy to combat the threat posed by special counsel Robert Mueller’s ongoing investigation into potential ties between Russia and the Trump campaign.

And the president’s lawyer, Rudy Giuliani, said that Mr Mueller recently shared a timetable that suggested that its probe could end by September 1 if Mr Trump were to sit for an interview in July, which is the legal team’s new working plan.

Mr Giuliani said he did not want a repeat of what happened in 2016, when FBI Director James Comey announced in the campaign’s final days that he was reopening the investigation into Hillary Clinton’s use of a private email server, a decision Democrats believe cost Mrs Clinton the race.

Mr Giuliani, the former mayor of New York, also said that Mr Mueller’s team indicated that the entire probe could end by September, not just its investigation into potential obstruction of justice.

“This would be the culmination of the investigation into the president,” Mr Giuliani said.

The special counsel’s office did not respond to a request for comment.

It is not certain if Trump will sit for an interview with Mr Mueller, but the president has publicly said he would.

Mr Giuliani said a decision on that would not be made until after Mr Trump’s summit with North Korean leader Kim Jong Un in Singapore, which is slated for June 12.

Rudy Giuliani (John Stillwell/PA)
Rudy Giuliani (John Stillwell/PA)

The former mayor said on Sunday the two sides “were getting closer” to agreeing on the parameters on a potential interview but would not put the odds of it happening at better than 50/50.

Mr Giuliani’s apparent attempt to publicly pressure Mr Mueller amid interview negotiations came just hours after Mr Trump’s demand for a new inquiry, which moved beyond his usual blustery accusations of institutional wrongdoing and into the realm of applying presidential pressure on the Justice Department, a move few of his predecessors have made.

Mr Trump made the order amid days of public venting about the special counsel investigation, which he has deemed a “witch hunt” that has yielded no evidence of collusion between his campaign and Russia.

In response, the Justice Department moved on Sunday to defuse a growing confrontation with the White House by asking its inspector general to expand an existing investigation into the Russia probe by examining whether there was any improper politically motivated surveillance.

The Justice Department probe had begun in March at the request of Attorney General Jeff Sessions and congressional Republicans.

Mr Sessions and the politicians had 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 Mr Trump.

Mr Horowitz said his office would look at those claims as well as communications between Steele and DOJ and FBI officials.

Robert Mueller, the special counsel probing Russian interference in the 2016 election (AP Photo/Andrew Harnik, File)
Robert Mueller, the special counsel probing Russian interference in the 2016 election (AP Photo/Andrew Harnik, File)

Trump did not elaborate on the promised “demand,” which he included in one of a series of tweets he sent throughout the day on Sunday.

On Saturday, he tweeted: “If the FBI or DOJ was infiltrating a campaign for the benefit of another campaign, that is a really big deal.”

He said only the release or review of documents the House Intelligence Committee is seeking from the Justice Department “can give conclusive answers”.