Washington: US president Donald Trump acknowledged on Sunday that his son met with Russians in 2016 at Trump Tower to get information on his election opponent Hillary Clinton, saying it was 'totally legal' and 'done all the time in politics.'

The Republican president had previously said the meeting was about the adoption of Russian children by Americans. Trump's morning Twitter post was his most direct statement on the purpose of the meeting.

In a post on Twitter, Trump also denied reports in the Washington Post and CNN that he was concerned his eldest son, Donald Trump Jr, could be in legal trouble because of the meeting with the Russians, including a lawyer with Kremlin ties.

He repeated that he had not known about the meeting in advance.

"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!" Trump said.

Political campaigns routinely pursue opposition research on their opponents, but not with foreign representatives from a country viewed as an adversary.

Special counsel Robert Mueller is examining whether Trump campaign members coordinated with Russia to sway the White House race in his favour. One part of the inquiry is focused on a June 9, 2016, meeting at Trump Tower between Donald Jr, other campaign aides, and a group of Russians.

An e-mail released by Donald Jr showed he had been keen on the meeting because his father's campaign was being offered potentially damaging information on Clinton.

Donald Jr said later he realized the meeting was primarily aimed at lobbying against the 2012 Magnitsky sanctions law, which led to Moscow denying Americans the right to adopt Russian orphans.

President Trump has repeatedly denied that his campaign worked with Moscow, saying "No Collusion!" Last week, however, he adopted his lawyers' tactics and insisted 'collusion is not a crime.'

Working with a foreign national with the intent of influencing a US election could violate multiple laws, according to legal experts.

CNN reported last month that Michael Cohen, the president's long-time personal lawyer, was willing to tell Mueller that Trump did know about the Trump Tower meeting in advance.

Trump's lawyers and the White House have given conflicting accounts about whether Trump was involved in crafting Donald Jr's response to a New York Times article last summer revealing the Trump Tower meeting with the adoptions rationale. Trump's lawyers acknowledged in a letter to Mueller's team in January 2018 that Trump dictated the response.

Trump has stepped up his public attacks on the Mueller probe since the first trial to arise from it began last week in Alexandria, Virginia, involving former Trump campaign chairman Paul Manafort.

The federal tax and bank fraud charges that Manafort faces are not related to the Trump campaign but Manafort's close relations with Russians and a Kremlin-backed Ukrainian politician are under scrutiny in the trial.

US intelligence agencies concluded last year that Russia interfered in the 2016 presidential election on Trump's behalf. Putin has denied his government was involved.

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