President Donald Trump had a second, previously undisclosed conversation with Russian President Vladimir Putin at a summit in Germany earlier this month.
White House spokesman Sean Spicer said Mr Trump and Mr Putin spoke during a world leaders' dinner at the G20 summit in Hamburg.
US intelligence officials have accused the Russian government of meddling in the 2016 election to help Mr Trump win.
Mr Trump and Mr Putin had a formal meeting that lasted more than two hours earlier that day.
It was not immediately clear how long the informal conversation lasted or what was discussed.
But Mr Trump defended the dinner in two angry tweets that noted the dinner had been on his public schedule.
He wrote: "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 few minutes later he added: "The Fake News is becoming more and more dishonest! Even a dinner arranged for top 20 leaders in Germany is made to look sinister!"
National Security Council spokesman Michael Anton said: "A conversation over dessert should not be characterised as a meeting."
The news came as Mr Trump announced his intention to nominate former Utah governor Jon Huntsman to be US ambassador to Russia.
If confirmed, the former 2012 Republican presidential candidate would take over the post amid the investigations into contacts between Russian officials and the Trump campaign.
Mr Huntsman has twice served as an ambassador. He was the nation's top diplomat to Singapore under President George HW Bush. He then served in that role in China under President Barack Obama before returning to the US to run for president.
Earlier, it emerged that a Russian developer's representative was the eighth attendee at a Trump Tower campaign meeting arranged by Mr Trump's oldest son.
In emails, Donald Trump Jr enthusiastically agreed to the meeting with Russian lawyer Natalia Veselnitskaya and others after he was promised dirt on his father's rival, Democrat Hillary Clinton.
The special counsel investigating possible Trump campaign ties to Russia wants more information about the meeting.
Officials from the office of Special Counsel Robert Mueller have contacted a lawyer for Ike Kaveladze, who also goes by the name Irakly Kaveladze, lawyer Scott Balber told The Washington Post and CNN.
It is the first public indication that Mr Mueller is probing the June 2016 gathering in Trump Tower.
Mr Kaveladze works for a Russian developer who once joined with Mr Trump to bring the Miss Universe pageant to Moscow.
Mr Balber told the media outlets that Mr Kaveladze works for developers Emin and Aras Agalarov and was there to represent them. The father and son, who worked with Mr Trump on the pageant in 2013, were named in the emails that promised damaging information on Mrs Clinton.
Ms Veselnitskaya has denied she works for the Russian government and said the meeting focused on US-Russian adoption policies and a sanctions law.
Mr Balber said Mr Kaveladze attended the meeting to help translate, but did not have to because Ms Veselnitskaya brought her own translator.
The meeting was also attended by Mr Trump's son-in-law Jared Kushner, then-campaign chairman Paul Manafort, music publicist Rob Goldstone and Rinat Akhmetshin, a prominent Russian-American lobbyist and former Soviet military officer.
Mr Trump Jr scheduled the gathering after Mr Goldstone, a British publicist for Emin Agalarov, said Ms Veselnitskaya might have damaging information on Mrs Clinton she could share.
Senator Mark Warner of Virginia, the senior Democrat on the Senate Intelligence Committee, said his panel wanted to talk to the meeting's attendees.
He said: "I doubt if this individual who had a history of setting up thousands of fake accounts in Delaware was really there to talk about Russian adoptions."
Mr Warner appeared to be referring to a 2000 New York Times story about Mr Kaveladze.
It identified him as running a company cited in a Government Accountability Office report for laundering $1.4bn in wealthy foreigners' funds via US banks using thousands of Delaware corporations. No criminal charges were filed in the case.