Congress is investigating Donald Trump over a potential conflict of interest involving increased US military spending at Prestwick airport near Glasgow and visits to his Trump Turnberry resort, according to reports.

The house oversight committee has been investigating whether increasing expenditure at the airport and allegations of US military personnel being offered discount deals at Trump’s Ayrshire golf resort represent a violation of the US constitution, Politico reported.

Elijah Cummings, the chairman of the committee, wrote in June to the then acting secretary of defence, Patrick Shanahan, to raise “serious conflict of interest concerns” about Trump making money from military trips to Scotland. According to the news website, the letter said US military spending at Prestwick, the closest airport to the resort, appeared to have “increased substantially since the election”.

It follows reports of the airport offering “cut-price rooms for select passengers and crew”, along with free rounds of golf at Turnberry for US military staff and civilian air crews. The air force confirmed that seven crew from a C-17 aircraft stayed at Turnberry in March.

The committee is demanding to see all the financial documents from the US Department of Defense relating to Trump Turnberry, in addition to details of all communications between the two parties. However, reports from the US suggest there has not yet been any response.

“Two years before the 2016 election, President Trump spent hundreds of millions of dollars to purchase and renovate the Turnberry golf course in Scotland,” said the letter, which was also signed by the Democrat congressman, Jamie Raskin. “To date, the property has continued to suffer financial losses and has not turned a profit for the president of his companies.

“The airport closest to the Trump Turnberry golf course – Glasgow Prestwick airport – has been viewed as integral to the golf course’s financial success, yet it too has lost millions of dollars every year since its purchase by the Scottish government in 2013.”

It cited Defence Logistics Agency records showing 629 fuel purchase orders worth $11m since October 2017. “Given the president’s continued financial stake in his Scotland golf courses, these reports raise questions about the president’s potential receipt of US or foreign government emoluments in violation of the US constitution and raise other serious conflict of interest concerns.

“In order to fulfil its responsibilities under the constitution, the committee is seeking information related to expenditures at both Prestwick airport and the president’s nearby Scotland golf resorts.”

CNN reported that a Democratic committee aide said the defense department had not cooperated with the investigation or produced any of the requested documents.

The US defense department said in a statement to the New York Times that a stopover of a US air force C-17 and its crew at Trump Turnberry in March 2019 had not been unusual, and they had “used the closest available and least expensive accommodations to the airfield within the crew’s allowable hotel rates”.

Both Trump Turnberry and the airport have operated at significant losses in recent years, with the Scottish government putting Prestwick up for sale after taking it into public ownership in 2013.