After his much-publicised two-and-a-quarter-hour meeting on July 7 with Russian President Putin in Hamburg, President Trump chatted informally with the Russian leader for up to an additional hour later the same day, the Washington Post reported.
The second meeting, undisclosed at the time, took place at a dinner for G-20 leaders, a senior administration official said.
During the meal, Trump left his own seat to occupy a chair next to Putin. Trump approached alone, and Putin was attended only by his official interpreter, the report said.
"The insinuation that the White House has tried to 'hide' a second meeting," it said, "is false, malicious and absurd."
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"All the leaders" circulated around the room throughout the dinner, and "President Trump spoke with many leaders," the statement said. "As the dinner was concluding," it said, Trump spoke "briefly" with Putin, who was seated next to first lady Melania Trump.
The two leaders' relationship is under intense scrutiny amid allegations of collusion between Russia and the Trump campaign during the US presidential polls.
US intelligence believe Russia helped Trump in last year's presidential election, something denied by Moscow. Trump has rejected any collusion.
Trump also said that Putin was unlikely to have preferred him over Hillary Clinton as president, since his goals include taking international energy business away from Russia and building up the US military.
The New York Times said the undisclosed Trump-Putin meeting raises new questions about their relationship.
"The intimate dinner conversation, of which there is no official US government record, because no American official other than the president was involved, is the latest to raise eyebrows. Foreign leaders who witnessed it later commented privately on the oddity of an American president flaunting such a close rapport with his Russian counterpart," the daily commented.
The absence of American translator in the meeting was breach of national security protocol, he said.
"Never in my life as a political scientist have I seen two countries -major countries -with a constellation of national interests that are as dissonant, while the two leaders seem to be doing everything possible to make nice and be close to each other," Bremmer told Bloomberg.
"Pretty much everyone at the dinner thought this was really weird, that here is the president of the US, who clearly wants to display that he has a better relationship personally with President Putin than any of us, or simply doesn't care," Bremmer was quoted as saying by The New York Times.
"They were flummoxed, they were confused and they were startled," he said.
But in a series of two tweets, Trump slammed the media.
"Fake News story of secret dinner with Putin is "sick." All G20 leaders, and spouses, were invited by the Chancellor of Germany," the US president said.
"Press knew about it. The Fake News is becoming more and more dishonest! Even a dinner arranged for top 20 leaders in Germany is made to look sinister!" Trump said in another tweet.