An unnamed donor is giving $130m (£100m) to the Pentagon to help pay US troops during the government shutdown, sparking ethics concerns.

US defence officials confirmed the gift, which President Donald Trump said will make up for shortfalls in paying the country's 1.32m service members, but declined to identify the donor.

Trump provided a few clues, though, saying on Saturday that he was "a big supporter of mine" and a US citizen.

The government shut down more than three weeks ago, after lawmakers failed to approve a funding deal. The Trump administration paid troops last week by shifting $8bn from military research, but it is unclear what will happen on the next payday at the end of the month.

Now in it's 25th day, the shutdown is poised to become one of the longest in US history.

"The donation was made on the condition that it be used to offset the cost of service members' salaries and benefits," Pentagon spokesman Sean Parnell told reporters on Friday, adding that the money was allowed under the department's "general gift acceptance authority".

Trump had indicated a donation would be made on Thursday, while also declining to identify the donor, saying "he doesn't really want the recognition".

However, the New York Times has identified the benefactor as Timothy Mellon, a billionaire and major financial backer of Trump.

Mr Mellon is the heir to his influential family's $15bn (£11bn) fortune, according to Forbes, and was heavily involved in the US railway industry before recently becoming more politically active and donating $50m to a group supporting Trump.

Before leaving for Asia on Saturday, Trump told reporters at the White House that the donor was a "great gentleman", a "patron", a "patriot" and a "wonderful man" who "doesn't want publicity".

"He prefers that his name not be mentioned, which is pretty unusual in the world I come from," Trump said. "In the world of politics, you want your name mentioned."

"He put up $130 million in order to make sure - that's a lot of money - in order to make sure that the military got paid, and he's a big supporter of mine," he continued.

The funds amount to around $100 per servicemember.

While the White House was able to reallocate defence funds to pay troops last week, it remains to be seen what will happen on the next payday for the military, on 31 October. Congress has been unable to pass a bill that would keep troops being paid during the shutdown.

Most government workers have been furloughed, or in some cases, told to work without pay while the shutdown lasts.

According to Defence Department rules, donations over $10,000 must be reviewed by ethics officials "to determine whether the donor is involved in any claims, procurement actions, litigation, or other particular matters involving the Department that must be considered prior to gift acceptance".

Donations from non-American citizens require additional scrutiny.

While unusual, the Pentagon does sometimes accept gifts from donors, but they normally must be given to fund specific projects, such as a school, hospital, library, museum, or cemetery.

Critics questioned why the US military would accept anonymous funds.

"Using anonymous donations to fund our military raises troubling questions of whether our own troops are at risk of literally being bought and paid for by foreign powers," said Delaware Senator Chris Coons, the top Democrat on the Senate's defense appropriations subcommittee.