Special counsel Robert Mueller has impaneled a grand jury to investigate Russias interference with the 2016 presidential election, The Wall Street Journal reported Thursday -- an important step toward potential criminal charges.

The newspaper, 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."

The establishment of a grand jury will allow Mueller -- a former FBI director -- to subpoena documents and get testimony under oath, as well as issue possible indictments.

Outgoing White House aide Sean Spicer and Justice Department spokeswoman Sarah Isgur Flores both declined to comment on the report.