Donald Trump Jr.'s wife, Bettina Trump, confirmed that their May wedding was paid for in part by an ally of Russian President Vladimir Putin.

A post on her Instagram account - attributed to the couple - said that two nights of their three-day wedding festivities on a private island in the Bahamas were partially funded by Umar Kremlev, a Russian oligarch with close ties to Putin.

The statement was released Monday after investigative journalism outlet ProPublica published a story detailing that Kremlev paid "hundreds of thousands of dollars" for the celebration.

Donald Trump Jr., one of the US president's sons, is a private citizen, so it's not illegal for him to accept gifts from a foreign national.

"Our dear friend Umar very generously hosted two incredible nights of celebrations for us AFTER our wedding. It was an extraordinarily generous wedding gift from a friend, and something for which we were and remain incredibly grateful," the couple said in the statement.

But ethics watchdog groups said it still presented the appearance of a conflict of interest for the president's son to accept lavish gifts from someone associated with a foreign head of state - even if he is not a US government employee.

"People don't just give hundreds of thousands of dollars to a random acquaintance without wanting something," said Jordan Libowitz, vice president of communications for Citizens for Responsibility and Ethics in Washington (CREW).

"You can see how [Putin] would enjoy all this press about how people in his circle are cozying up to people in the president's circle."

According to the ProPublica article, the couple was married on a private island in the Bahamas, then held a two-day celebration on a separate island that rents for approximately $100,000 per night. Kremlev also paid other costs for the party, including a roughly $70,000 fireworks display, the report found.

Kremlev runs the International Boxing Association (IBA), an organization funded by the state-backed Gazprom energy company. The Olympic Committee suspended the group in part over its failure to report its ties to the Russian energy giant.

Earlier this year Kremlev was part of a delegation that visited China with Putin. The Russian president also gave Kremlev the Order of Friendship, a top state honor, according to ProPublica.

A spokesperson for Trump Jr. told the news outlet that the president's son and Kremlev were personal friends and did not have "a business relationship."

An ally of Trump Jr. referred the BBC to the Instagram statement.

In a statement to ProPublica, Kremlev's office said "Mr. Kremlev and Mr. Trump Jr have a friendly relationship." The statement said the men met a few years ago.

Kremlev attended part of the wedding, the outlet reported. Other attendees included one of the US president's daughters, Ivanka Trump, and her husband, Jared Kushner, who has worked for the US on diplomatic negotiations with Iran, Russia and in the Middle East.

The Trump campaign and family faced controversy for potential ties to Russia during the 2016 presidential election, resulting in a special counsel investigation. During the campaign Donald Trump Jr. brokered a meeting at Trump Tower with a Russian operative that was closely scrutinized during the probe.

Libowitz, of CREW, said either the president, who is a billionaire, or Trump Jr. -- who is worth $300 million, according to Forbes -- could have paid for the full wedding to avoid a conflict of interest.

"His father is worth billions he could have picked up the tab. Clearly they just didn't want to pay for it," Libowitz said. "They don't really care about the ethical ramifications."