The US president’s son, Donald Trump Jr, has admitted meeting a Kremlin-linked Russian lawyer last year.
The encounter is thought to be the first confirmed private meeting between a Russian national and a member of Donald Trump’s inner circle.
A special prosecutor is investigating whether Trump associates colluded with alleged Russian efforts to influence last November’s US election.
Both Mr Trump Jr and the lawyer say the campaign was not discussed.
Mr Trump Jr was accompanied by the president’s son-in-law Jared Kushner, and then-campaign head Paul J Manafort, meeting Natalia Veselnitskaya at New York’s Trump Tower on 9 June, two weeks after Donald Trump secured the Republican nomination.
Mr Trump Jr said in a statement that they discussed a suspended programme for Americans to adopt Russian children.
He said it “was not a campaign issue at that time and there was no follow-up”.
Mr Kushner’s lawyer said he had previously disclosed the meeting on security clearance forms.
President Vladimir Putin suspended the adoption programme in 2012 after the US Congress voted in a law to allow the US to withhold visas and freeze financial assets of Russian officials thought to have been involved with human rights violations.
Ms Veselnitskaya, who played a key role campaigning against the law, said “nothing at all was discussed about the presidential campaign. “I have never acted on behalf of the Russian government and have never discussed any of these matters with any representative of the Russian government.”
Last week Mr Trump said interference in the election “could well have been” carried out by countries other than Russia and interference “has been happening for a long time”.
