WASHINGTON (WASHINGTON POST) - The White House has approved eight flights on military jets for Treasury Secretary Steven Mnuchin to travel to destinations from West Virginia to Italy, an investigation by the agency's watchdog found, a use of government planes that was legal but poorly justified by the Trump administration.

In a review of nine requests for Mr Mnuchin to fly on government planes - to business meetings in London, Canada, Miami, New York and elsewhere - the office of Treasury Department Inspector General Eric Thorson found that Trump officials cut corners as they tried to justify the trips, as federal travel regulations require.

Mr Thorson's counsel Rich Delmar, who investigated the flights, also warned the administration that although the travel was legal, "just because something is legal doesn't make it right".

"Even when the criteria (of federal regulations) allow for the use of Government-owned, rented, leased, or chartered aircraft, departments and agencies should still consider whether commercial air travel is a more appropriate use of taxpayer resources," Mr Delmar wrote in a 21-page report issued late on Thursday.

"With few exceptions, the commercial air system used by millions of Americans every day is appropriate, even for very senior officials."

The review opens a window on a cost of government business that, while justified in Mr Mnuchin's case because of his need for secure communications aboard planes, is nonetheless pricey.

The tab for him and his staffers to fly on a C-32 jet to Bari, Italy, in May to attend the Group of Seven finance ministers conference ran to US$314,442 (S$429,791) at an hourly rate of US$15,944, for example.

When Mr Mnuchin flew from Trump Tower in New York back to Washington after President Donald Trump summoned Cabinet members for his speech after the violence in Charlottesville, Virginia, in August, the 179-mile trip cost taxpayers US$15,112.

And when Mr Mnuchin, his wife, Senate Majority Leader Mitch McConnell and other officials flew to Louisville and Fort Knox in Kentucky in August on an Air Force C-37B to visit the US Mint and then view the solar eclipse, the government paid US$26,900.

Mr Mnuchin reimbursed the government for the cost to fly his wife, Ms Louise Linton.

Mr Mnuchin is one of five members of Mr Trump's Cabinet whose travel on military or private charter flights has come under scrutiny from the public and lawmakers on Capitol Hill.

Inspectors general have opened investigations into the travel habits of former health and human services secretary Tom Price - who was forced to resign last week after racking up at least US$500,000 in taxpayer-funded charter flights - Environmental Protection Agency Administrator Scott Priutt, Interior Secretary Ryan Zinke and Veterans Affairs Secretary David Shulkin.

The Washington Post reported last week that Mr Shulkin took a 10-day trip to Europe in July with his wife and two top aides that mixed meetings on veterans' health issues with four days of sightseeing at taxpayer expense. His wife's airfare and meals were covered by the government.

The Mnuchin review was prompted by news reports about Mr Mnuchin's flights, which included a request to have a government plane fly him and Ms Linton to Europe this summer for their honeymoon. That request, which Treasury officials said was made to guarantee access to secure communications during the trip, was withdrawn before any flight.

A Treasury spokesman said in an e-mailed statement that the review "confirmed that each of the Secretary's uses of government aircraft was approved by the White House".

"The Department followed the same approval procedures and provided the White House the same level of justification as in previous administrations, including the Obama Administration," the statement said. The agency said it would incorporate enhancements the report calls for in future travel requests.

But Mr Delmar said he found a disconnect between the rules requiring government officials to document why they need to travel by military jet and whether the trips meet the standard of "White House Support Missions" and "the actual amount of proof provided by Treasury and accepted by the White House in justifying these trip requests".

Mr Delmar found that the proof in Mr Mnuchin's case cut corners.

"My summaries show that in almost all cases a single boilerplate statement constituted the whole analysis and justification for designation and use of military aircraft, despite the fact that (the accepted rule) clearly calls for a more rigorous and complete provision of facts and arguments," Mr Delmar wrote.

The language that was frequently used read: "Due to scheduling, logistics,and communications needs, the use of reimbursable military aircraft, preferably a C-40, is requested."

To be eligible as a "White House Support Mission", the president must have specifically directed that the trip be taken and the official must justify why a commercial airline is not an option. The agency must explain why government planes are more cost-effective than commercial, if, for example, the number of people travelling is large enough to justify the extraordinary cost of a government flight.

The watchdog found "no detailed analysis" along these lines.

A flight to Palm Beach, Florida, by Mr Mnuchin's top aide Eli Miller - on the private jet of a hedge fund billionaire and major contributor to Republican campaigns - is under separate review by the inspector general's office after The Post reported on the trip.