The House Committee on Oversight and Reform is not getting cooperation from the Pentagon in its probe of military sleepovers at President Trump's golf resort in Scotland, a senior Democratic aide told NBC News.

The conflict-of-interest probe, first reported Friday by Politico, seeks Department of Defense documents and other information related to post-election military stops in Prestwick, Scotland, not far from Trump Turnberry, according to a letter co-signed by oversight Chairman and U.S. Rep. Elijah Cummings, D-Md.

"The Defense Department has not produced a single document in this investigation," the aide said. "The committee will be forced to consider alternative steps if the Pentagon does not begin complying voluntarily in the coming days."

Presidential candidate Donald Trump answers questions from the media at a press conference during a visit to his Scottish golf course Turnberry on July 20, 2015. Russell Cheyne / Action Images via Reuters file

The letter, dated June 21 but unknown to the public until this week, states that a look at Defense Logistics Agency data shows that, since October 2017, taxpayers have spent $11 million on 629 fuel purchase orders at Glasgow Prestwick Airport in Scotland.

"The jet fuel purchases appear to have supported various Defense Department missions," the letter, also signed by Rep. Jamie Raskin, D-Md., states.

The ailing airport has lost millions of dollars in revenue in recent years, according to the letter, and its existence is crucial to the golf resort's survival. The airport has offered discounts and free rounds of golf to members of the U.S. military, the letter states, citing the Guardian.

A person familiar with at least one of the flights, an Air National Guard supply delivery to Kuwait, told Politico the Trump Turnberry sleepovers to and from the Middle East were unusual because the C-17 cargo plane usually stopped at U.S. or allied military bases, Ramstein Air Base in Germany or Naval Station Rota in Spain.

The Politico report also says jet fuel is usually less expensive at bases. An Air Force official told the publication that staying at a luxury resort was not normal for on-duty service members traveling at taxpayers' expense.

The House Oversight letter, citing the Washington Post, states losses at Trump's Scottish resort amounted to $4.5 million in 2017. Politico reported in June that revenue increased by $3.1 million the next year.

The probe of government spending at the president's Scottish resort was revealed on the same day Cummings announced the Oversight Commitee is seeking more information about Vice President Mike Pence's stay earlier in the week at Trump's Irish golf resort.

Pence was with his family during the stay at Trump International Golf Links and Hotel in Doonbeg, Ireland. The resort is about 180 miles away from Dublin, where the vice president met with Irish officials.

Pence's chief of staff, Marc Short, said the detour to Doonbeg was suggested by the president. But the vice president's office subsequently issued a statement saying the trip to Doonbeg, where Pence has familial ties, was "solely" the idea of the vice president and his staff.

Short told a New York Times reporter the vice president would cover the costs for his family's stay.