WASHINGTON (AFP) - A Dutch lawyer with direct knowledge of contacts between Russian intelligence and a top official in Donald Trump's campaign became the first person sentenced in special prosecutor Robert Mueller's sprawling investigation on Tuesday (April 3).

Alex van der Zwaan was sentenced to 30 days in jail and a US$20,000 fine after pleading guilty to lying about his contacts with former campaign deputy Rick Gates and a former Russian intelligence official.

Van der Zwaan, a Dutch national with Russian roots and son-in-law of a prominent Russian tycoon, was a lawyer in London for Skadden, Arps, Slate, Meagher & Flom in 2012 when he carried out work for the Ukraine government through Gates and Gates' close associate Paul Manafort.

In 2016, after Manafort became chairman of Trump's election campaign and took Gates as his deputy, van der Zwaan and Gates both had communications with a person they knew as a former official of Russia's GRU intelligence agency, prosecutors said.

According to the FBI, court documents say, that individual - identified as "Person A" - "has ties to a Russian intelligence service and had such ties in 2016".

Van der Zwaan lied to investigators on several occasions about his contacts with both Gates and "Person A," prosecutors said.

After his lies were called out, van der Zwaan began cooperating with investigators, including sharing recordings of his conversations with Gates, "Person A" and with a senior Skadden partner.

Case adds to pressure on Trump

While the details of those conversations remain secret, court documents suggest they support the idea of more extensive contacts between the Trump campaign and Russia during the election than the White House has admitted.

Van der Zwaan faced up to five years in jail. He earned a light sentence based on his cooperation with Mueller's probe, which is focused on whether there was collusion between the Trump campaign and Russian interference in the 2016 presidential election.

US intelligence agencies say President Vladimir Putin himself was behind a hacking and disinformation effort to disrupt the election and boost Trump's chances of winning. Mueller's team has kept most of the details of what van der Zwaan told them secret.

So it remains unclear whether his information supports collusion allegations.

But the case adds to the rising pressure on Trump, his family and top aides from the probe.

Mueller's team, formally known as the Office of the Special Counsel, said in a filing that "van der Zwaan is in an unusual position of having information related to the office's investigation that is not widely known - including information that he knows first-hand due to his role in the conduct the office is investigating."

Manafort-Moscow links eyed

Mueller, a former FBI director and federal prosecutor, was named last May to investigate possible collusion between Trump campaign and Russia.

So far his team has issued indictments of 19 people, including Gates and Manafort, and 13 Russian nationals, including a close associate of President Vladimir Putin, who were involved the effort to use social media to interfere in the US election.

With specific authorization from the US Justice Department, Mueller has probed deeply into the work Gates and Manafort did for Russia-allied former Ukraine president Viktor Yanukovych between 2006 and 2014.

The investigation has reportedly focused on Konstantin Kilimnik, a former agent of Russia's GRU intelligence agency who was based in Kiev and worked with Manafort and Gates during that period.

Kilimnik has told US media that those contacts continued when Manafort took the lead in Trump's campaign in 2016, including as a go-between for Manafort and Russian billionaire Oleg Deripaska, a close Putin associate.

The Washington Post reported last year that in a July 2016 email Manafort offered Deripaska briefings on the presidential race during the campaign.

In a court filing on Monday, Mueller defended his expansion of the probe to cover Manafort's Ukraine work.

One of the first charged by Mueller, Manafort has pleaded not guilty to multiple charges of money laundering, tax evasion, bank fraud and lying. Facing the same charges, in February Gates reached a plea deal with Mueller's prosectors.

He agreed to cooperate in exchange for pleading guilty to one charge of lying and one count of conspiracy to defraud the United States, relating to unreported offshore bank accounts.

Gates pleaded guilty on Feb 23 to conspiring to defraud the United States and lying to Mueller's office, and is now cooperating with the probe.