Donald Trump's son-in-law and close adviser, Jared Kushner, had at least three previously undisclosed contacts with the Russian ambassador to the United States during and after the 2016 presidential campaign, seven current and former US officials said.

Those contacts included two phone calls between April and November last year, two of the sources said. By early this year, Mr Kushner had become a focus of the FBI investigation into whether there was any collusion between the Trump campaign and the Kremlin, said two other sources - one current and one former law enforcement official.

Mr Kushner initially had come to the attention of FBI investigators last year as they began scrutinising former national security adviser Michael Flynn's connections with Russian officials, the two sources said.

While the FBI is investigating Mr Kushner's contacts with Russia, he is not currently a target of that investigation, the current law enforcement official said.

The new information about the two calls as well as other details uncovered by Reuters shed light on when and why Mr Kushner first attracted FBI attention and show that his contacts with Russian envoy Sergei Kislyak were more extensive than the White House has acknowledged.

NBC News reported on Thursday that Mr Kushner was under scrutiny by the FBI, in the first sign that the investigation, which began last July, has reached the President's inner circle.

The FBI declined to comment, while the Russian embassy said it was policy not to comment on individual diplomatic contacts. The White House did not respond to a request for comment.

Multiple attempts to obtain comment from Mr Kushner or his representatives were unsuccessful.

In March, the White House said that Mr Kushner and Mr Flynn had met Mr Kislyak at Trump Tower in December to establish 'a line of communication.' Mr Kislyak also attended a Trump campaign speech in Washington in April 2016 that Mr Kushner attended. The White House did not acknowledge any other contacts between Mr Kushner and Russian officials.

Before the election, Mr Kislyak's undisclosed discussions with Mr Kushner and Mr Flynn focused on fighting terrorism and improving US-Russian economic relations, six of the sources said. Former President Barack Obama imposed sanctions on Russia after it seized Crimea and started supporting separatists in eastern Ukraine in 2014.

After the 8 November election, Mr Kushner and Mr Flynn also discussed with Mr Kislyak the idea of creating a back channel between Trump and Russian President Vladimir Putin that could have bypassed diplomats and intelligence agencies, two of the sources said. Reuters was unable to determine how those discussions were conducted or exactly when they took place.

Reuters was first to report last week that a proposal for a back channel was discussed between Mr Flynn and Mr Kislyak as Mr Trump prepared to take office. The Washington Post was first to report on Friday that Mr Kushner participated in that conversation.

Separately, there were at least 18 undisclosed calls and emails between Trump associates and Kremlin-linked people in the seven months before the 8 November presidential election, including six calls with Mr Kislyak, sources told Reuters earlier this month. Two people familiar with those 18 contacts said Flynn and Kushner were among the Trump associates who spoke to the ambassador by telephone. Reuters previously reported only Flynn's involvement in those discussions.

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Six of the sources said there were multiple contacts between Mr Kushner and Mr Kislyak but declined to give details beyond the two phone calls between April and November and the post-election conversation about setting up a back channel. It is also not clear whether Mr Kushner engaged with Mr Kislyak on his own or with other Trump aides.

FBI scrutiny of Mr Kushner began when intelligence reports of Mr Flynn's contacts with Russians included mentions of US citizens, whose names were redacted because of US privacy laws. This prompted investigators to ask US intelligence agencies to reveal the names of the Americans, the current US law enforcement official said.

Mr Kushner's was one of the names that was revealed, the official said, prompting a closer look at the President's son-in-law's dealings with Mr Kislyak and other Russians.

FBI investigators are examining whether Russians suggested to Mr Kushner or other Trump aides that relaxing economic sanctions would allow Russian banks to offer financing to people with ties to Mr Trump, said the current US law enforcement official.

The head of Russian state-owned Vnesheconombank, Sergei Nikolaevich Gorkov, a trained intelligence officer whom Mr Putin appointed, met Mr Kushner at Trump Tower in December. The bank is under US sanctions and was implicated in a 2015 espionage case in which one of its New York executives pleaded guilty to spying and was jailed.

The bank said in a statement in March that it had met with Mr Kushner along with other representatives of US banks and business as part of preparing a new corporate strategy.

Officials familiar with intelligence on contacts between the Russians and Trump advisers said that so far they have not seen evidence of any wrongdoing or collusion between the Trump camp and the Kremlin. Moreover, they said, nothing found so far indicates that Mr Trump authorised, or was even aware of, the contacts.

There may not have been anything improper about the contacts, the current law enforcement official stressed.

Mr Kushner offered in March to be interviewed by the Senate Intelligence Committee, which is also investigating Russia's attempts to interfere in last year's election.

The contacts between Trump campaign associates and Russian officials during the presidential campaign coincided with what US intelligence agencies concluded was a Kremlin effort through computer hacking, fake news and propaganda to boost Mr Trump's chances of winning the White House and damage his Democratic opponent, Hillary Clinton.