A US watchdog has requested documents to shed light on the reasons behind the use of a government plane by the Treasury Secretary Steve Mnuchin and his wife Louise Linton.

The Citizens for Ethics and Responsibility in Washington (CREW) has filed an FOIA (Freedom Of Information Act) request for the cost details of a trip to Lexington, Kentucky on the 21 August, the same day as the solar eclipse.

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Questions have been raised as to whether the couple used the government plane to get a better view of the eclipse which reached totality over parts of the Bluegrass state.

CREW issued a statement on their website saying: "The requested records would shed light on the justification for Secretary Mnuchin's use of a government plane, rather than a commercial flight, for a trip that seems to have been planned around the solar eclipse and to enable the Secretary to secure a viewpoint in the path of the eclipse's totality.

"At a time of expected deep cuts to the federal budget, the taxpayers have a significant interest in learning the extent to which Secretary Mnuchin has used government planes for travel in lieu of commercial planes, and the justification for that use."

The trip has already sparked controversy after Louise Linton posted a now deleted image of the trip on Instagram. A user of the photo-sharing app said: "Glad we could pay for your little getaway. #deplorable."

Linton replied saying: "Pretty sure the amount we sacrifice per year is a lot more than you'd be willing to sacrifice if the choice was yours. You're adorably out of touch."

After facing a public backlash to the comment, Linton issued an apology: "I apologise for my post on social media yesterday as well as my response."

Steven Mnuchin and Louise Linton
Steven Mnuchin (L) and his financee Louise Linton watch as US President Donald Trump speaks during Mnuchin's swearing-in ceremony as the next treasury secretary in the Oval Office of the White House on February 13, 2017 MANDEL NGAN/AFP/Getty Images