Donald Trump and Vladimir Putin had an additional, previously undisclosed chat at this month's G20 summit in Hamburg, the White House acknowledged on Tuesday.

After a brief greeting at the start of the two-day summit and a two-hour bilateral meeting with their foreign ministers on July 7, Trump and Putin also chatted over dinner on the final night of the summit, an official said.

"There was a couples-only social dinner at the G20," a White House official told AFP. "Toward the end, the president spoke to Putin at the dinner."

The disclosure has raised questions about what the pair talked about, who was present and why the meeting was not previously mentioned.

The Trump administration has been besieged by allegations that the president's closest advisors colluded with Russia to win the 2016 election.

Trump's eldest son Donald Jr recently released emails showing he held a meeting during the campaign with Kremlin-connected figures, hoping to get dirt on his dad's election rival Hillary Clinton.

Trump late Tuesday took to Twitter to blast the suggestion that there was anything improper about his dinner chat with Putin.

"Fake News story of secret dinner with Putin is 'sick.' All G20 leaders, and spouses, were invited by the Chancellor of Germany. Press knew!" he declared.

"The Fake News is becoming more and more dishonest! Even a dinner arranged for top 20 leaders in Germany is made to look sinister!"

A second White House official denied there was a "second meeting" between Trump and Putin, describing a "brief conversation at the end of a dinner."

"The insinuation that the White House has tried to 'hide' a second meeting is false, malicious and absurd," the official said.

"At the dinner, President Trump was seated between Mrs. Abe, wife of the Prime Minister of Japan, and Mrs. Macri, wife of the President of Argentina. Mrs. Trump was seated next to President Putin."

"During the course of the dinner, all the leaders circulated throughout the room and spoke with one another freely. President Trump spoke with many leaders during the course of the evening. As the dinner was concluding, President Trump went over to Mrs. Trump, where he spoke briefly with President Putin."

The White House also rejected concerns that Trump was not with any other US officials and used Putin's translator.

"Each couple was allowed one translator. The American translator accompanying President Trump spoke Japanese. When President Trump spoke to President Putin, the two leaders used the Russian translator, since the American translator did not speak Russian."