US President Donald Trump and Russian President Vladimir Putin had a previously undisclosed conversation during a dinner for G20 leaders at a summit earlier this month in Germany, a White House official said on Tuesday.

The two leaders held a formal two-hour bilateral meeting on July 7 in which Trump later said Putin denied allegations that he directed efforts to meddle in the 2016 US presidential election.

Trump's interactions with the Russian leader were scrutinised closely because of those allegations, which have dominated his first six months in the White House, and Trump's comments as a presidential candidate praising the former KGB spy.

Trump and Putin first met at the G20 during a gathering of other leaders, which was shown in a video. They later held the bilateral meeting, which was attended briefly by a pool of reporters.

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In the evening, both men attended a dinner with G20 leaders. Putin was seated next to US first lady Melania Trump. The US president went over to them at the conclusion of the dinner and visited with Putin, the official said. That conversation had not been previously disclosed.

"There was no 'second meeting' between President Trump and President Putin, just 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.

In a tweet late on Tuesday, Trump said: "Fake News story of secret dinner with Putin is "sick." All G 20 leaders, and spouses, were invited by the Chancellor of Germany. Press knew!"

'A breach of national security protocol'

News of the conversation, first reported by Ian Bremmer, the president of political risk consultancy Eurasia Group, could raise renewed concern as congress and a special counsel investigate allegations by US intelligence agencies that Russia interfered to help Trump, a Republican, win the presidency.

Trump says there was no collusion and Russia denies interference in the election.

Bremmer said Trump got up from his seat halfway through dinner and spent about an hour talking "privately and animatedly" with Putin, "joined only by Putin's own translator".

The lack of a US translator raised eyebrows among other leaders at the dinner, said Bremmer, who called it a "breach of national security protocol".

Al Jazeera's Heidi Zhou-Castro, reporting from Washington, DC, said the White House was trying to downplay the encounter as a conversation over dessert, rather than a meeting.

"But the reason that this has stirred up so much attention is because the relationship between Putin and Trump is under close scrutiny. The US intelligence agency has determined that Moscow attempted to interfere with the US election in favour of Donald Trump, and now multiple entities are investigating whether Trump's campaign colluded with Russia," she said.

"Now, the revelation about this 'meeting', which was not revealed until now, adds to the perception that Trump and Putin may have a closer relationship than previously thought."

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The White House official said the leaders and their spouses were only permitted to have one translator attend the dinner. Trump sat next to Japanese Prime Minister Shinzo Abe's wife. His translator spoke Japanese.

"When President Trump spoke to President Putin, the two leaders used the Russian translator, since the American translator did not speak Russian," the official said.

A US official who was briefed by some of his counterparts about the encounter said some of the leaders who attended the dinner were surprised to see Trump leave his seat and engage Putin in an extended private conversation with no one else from the US side present.

"No one is sure what their discussion was about, and whether it was purely social or touched on bilateral or international issues," the official said.

Lawrence Korb, senior fellow at the Center for American Progress, told Al Jazeera that other world leaders would likely look at this encounter and try to get Trump on his own in the future to have this kind of private conversation.

"Because they can get him to commit to things that, on the surface, may sound reasonable, but when you dig underneath you find out really that this is against American interests," he said.

"The relations with countries are very complicated, nations don't have permanent friends or enemies - they have permanent interests. And so, we don't know what Putin was able to get [Trump] to agree to ... those are the type of things that I think the diplomats rightly worry about."