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The US Air Force (USAF) has ordered a review of its guidance on overnight accommodation for flight crews.

It has emerged that some personnel have been staying at one of President Donald Trump's Scottish golf resorts.

There has been an increase in the number of US military flights stopping at Prestwick Airport, Scotland, near the resort, since he took office.

A US congressional committee is investigating Mr Trump for a potential conflict of interest over the matter.

Air Force chiefs have "directed Air Mobility Command [AMC, which oversees all Air Force transport around the world] to review all guidance pertaining to selection of airports and lodging accommodations during international travel", according to a statement to the BBC from Brig Gen Edward Thomas.

The US Air Force said its crews had obeyed all the rules, but said "lodging at higher-end accommodations, even if within government rates, might be allowable but not advisable".

Brig Gen Thomas also explained the increased use of Prestwick airport in the last four years because of a number of key factors, including longer operating hours and standardisation of routing locations.

"Between 2015 and 2019, AMC Total Force aircraft stopped at Prestwick a total of 936 times (*659 overnight stays), including 95 (*40) in 2015, 145 (*75) in 2016, 180 (*116) in 2017, 257 (*208) in 2018 and 259 (*220) through August 2019," his statement added.

Media playback is unsupported on your device Media caption Donald Trump on the Turnberry golf course in 2014

Over the weekend, it emerged that the crew of a US C-17 military transport aircraft stayed at Trump Turnberry when it stopped at Prestwick en route to Kuwait earlier in the year.

Mr Trump has tweeted he did not know anything about the matter:

The Air Force has not said how many of its staff have stayed at the president's resort.

Democrats and critics argue such stays might enrich the president at taxpayers' expense as crews who land at the airport then go on to stay at the nearby Trump Turnberry resort.

The House Oversight and Reform Committee says expenditure at Prestwick airport has "increased substantially" since Mr Trump came into office.

The debt-ridden airport 34 miles (55km) from Glasgow has been fighting off closure.

It is said to be integral to the Trump business, which is also loss-making.

The committee's accusations are detailed in a letter to the Pentagon - which is dated to June, but was only revealed on the Politico website on Friday.

Citing Defence Logistics Agency (DLA) records, it said the US military had made 629 fuel purchase orders at the airport, totalling $11m (£9m), since October 2017.

It also alleges that certain military personnel have been offered "cut-price rooms" and free rounds of golf at the Trump Turnberry resort.

In a statement sent on Monday to the BBC, Glasgow Prestwick airport said: "Like all airports, we provide a full handling service for customers and routinely arrange overnight accommodation for visiting aircrew when requested. We use over a dozen local hotels, including Trump Turnberry, which accounts for a small percentage of the total hotel bookings we make.

"It's important to note that we do not pay for aircrew accommodation and take no commission from Trump Turnberry for any bookings made on behalf of our customers. All aircrew landing at Glasgow Prestwick settle their bills directly with the hotels involved and, contrary to some claims we have seen, we do not offer free rounds of golf at Trump Turnberry for any aircrew."

Who owns the airport?

Prestwick airport, south of Glasgow, is approximately 20 miles (30km) north of Trump Turnberry.

The Scottish government bought it for £1 in 2013, when it was facing closure.

In June, it was put up for sale. No buyer has been announced.

Amid rising debts, the airport has reportedly slashed its charges to try to retain business.