US President Donald Trump's eldest son has released an email chain showing he eagerly agreed to meet with a Russian government lawyer to obtain incriminating information on his father's Democratic rival Hillary Clinton as part of Moscow's support for the then Republican candidate.

Key points: The emails could provide ammunition in the probe looking into Russian-Trump collusion

The emails could provide ammunition in the probe looking into Russian-Trump collusion Collusion itself is not a crime but abetting criminal actions could be grounds for charges

Collusion itself is not a crime but abetting criminal actions could be grounds for charges Donald Trump Jr met with a Russian lawyer but has said they discussed other things

Donald Trump Jr met with a Russian lawyer but has said they discussed other things Sources say the committee investigating Russian interference are seeking the emails

"The Crown prosecutor of Russia … offered to provide the Trump campaign with some official documents and information that would incriminate Hillary and her dealings with Russia and would be very useful to your father," said the June 3, 2016, email to Donald Trump Jr from publicist Rob Goldstone.

"This is obviously very high level and sensitive information but is part of Russia and its Government's support for Mr Trump," according to the email posted by Mr Trump Jr on Twitter.

"If it's what you say I love it," Mr Trump Jr partly replied in the exchange, which he said represented the entire chain of his emails about the meeting, which eventually took place on June 9, 2016.

The email chain was between Donald Trump Jr, who posted it on Twitter, and Mr Goldstone, an intermediary who helped to arrange the meeting with the lawyer.

Mr Trump's son-in-law Jared Kushner and former campaign manager Paul Manafort are also included in sections of the email thread to set up the meeting.

Observers were quick to question why Mr Trump Jr would release hard evidence demonstrating potentially damning information about himself and his father's campaign links to Russia.

The email exchange where Mr Trump Jr says "I love it" in response to the offer. ( Twitter: @DonaldJTrumpJr )

The documents could provide ammunition for investigators who are probing whether there was collusion between the Kremlin and Mr Trump's presidential campaign.

"In retrospect, I probably would have done things a little differently," Mr Trump Jr said in an interview with Fox News.

"For me, this was opposition research."

The email exchange includes at least one error: Russia, which ceased to be a monarchy during its revolution, does not have a "crown prosecutor" but rather a prosecutor-general.

A spokesman for the prosecutor-general declined to comment immediately.

Natalia Veselnitskaya, the Russian who met with Mr Trump Jr, has said she is a private lawyer and has denied having Kremlin ties.

Following the email thread release, Mr Trump released a statement applauding his eldest son.

"My son is a high-quality person and I applaud his transparency," Mr Trump said in a brief statement read to reporters by White House spokeswoman Sarah Sanders.

Is this evidence of collusion with Russia, or not?

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Legal experts are divided on whether Mr Trump Jr's participation in the meeting with the Russian lawyer could lead to criminal liability.

Collusion in and of itself is not a crime. But if the younger Trump conspired or aided and abetted a criminal action, such as hacking into American computer networks, that could be grounds for criminal charges.

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Several lawyers also said the meeting could run afoul of federal election laws barring campaigns from accepting gifts or things of value from foreign nationals.

Mr Trump Jr hired a lawyer on Monday to represent him in the Russia-related investigations as prominent Republicans voiced concern about the meeting.

The emails do not at first glance appear to provide evidence of illegal activity, and Mr Trump Jr has said a meeting he later had with a Russian lawyer arranged via Mr Goldstone primarily involved discussion of US sanctions.

However, Mr Goldstone's statement that the promise of incriminating information on Mrs Clinton was "part of Russia and its Government's support for Mr Trump" provides new ammunition for federal and congressional investigators who are probing Russia's alleged interference in the 2016 US presidential campaign, in which Mr Trump beat Mrs Clinton.

Moscow has denied any interference, and Mr Trump says his campaign did not collude with Russia.

Special Counsel Robert Mueller is investigating the matter, as are US congressional committees, including the Senate and House of Representatives intelligence panels.

A Senate source said the Senate Intelligence Committee did plan to call the President's son to testify and that it was seeking documents from him.

The Republican chairman of the committee, Senator Richard Burr, declined to comment on its plans.

"I don't draw conclusions until the investigation is completed," Mr Burr told reporters.

Reuters/ABC