The Wall Street Journal , citing two unnamed sources familiar with the matter, reported that the grand jury had begun its work in the US capital Washington "in recent weeks."
The move is a sign that the sweeping federal investigation -- which includes allegations that Trump campaign officials coordinated with Russia to tilt the election -- is gathering pace and has entered a more concrete phase.
Trump has repeatedly denied the allegations of collusion, saying he is the victim of a political "witch hunt."
Outgoing White House aide Sean Spicer and Justice Department spokeswoman Sarah Isgur Flores both declined to comment on the report.
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