US President Donald Trump has admitted his son met a Russian lawyer in June 2016 "to get information on an opponent", but argues it was legal.
Donald Trump Jr's meeting with Kremlin-linked lawyer Natalia Veselnitskaya at Trump Tower could constitute a breach of US campaign rules, experts say.
And the meeting is being investigated by Special Counsel Robert Mueller as part of his inquiry into Russia's alleged role to help Mr Trump win.
President Trump denies any collusion.
He has called the ongoing investigations in the US "the greatest political witch hunt in history".
Russia has repeatedly denied claims it interfered in the November 2016 presidential elections, where Mr Trump defeated Democratic rival Hillary Clinton.
What did President Trump say?
On Sunday, US media including the Washington Post, CNN and AP reported that Mr Trump was worried Donald Trump Jr could be in legal trouble because of the 9 June 2016 meeting with Ms Veselnitskaya in New York. They cited several unnamed sources.
Mr Trump tweeted: "Fake News reporting, a complete fabrication, that I am concerned about the meeting my wonderful son, Donald, had in Trump Tower.
"This was a meeting to get information on an opponent, totally legal and done all the time in politics – and it went nowhere.
"I did not know about it!"
Why does his tweet matter?
Mr Trump's latest tweet appears to contradict a previous statement from the Trump camp about the meeting.
When the meeting was first reported by the New York Times, Donald Trump Jr said in a statement that he and Ms Veselnitskaya had mostly discussed a suspended programme for Americans to adopt Russian children.
However, he subsequently admitted he had agreed to the meeting after being told he would be offered information that would prove detrimental to Mrs Clinton. He also released the email exchange that brought about the meeting.