Special counsel Robert Mueller impaneled a grand jury in Washington on Thursday as the latest step in the investigation he is heading into alleged Russian interference in the 2016 presidential election, the Wall Street Journal reported, citing two unnamed sources with knowledge of the matter.

  The grand jury began working on the issue in recent weeks and its activities indicate that Mueller is stepping up his work on the Russia probe, one of the key elements of which is whether President Donald Trump's election campaign colluded with the Kremlin to harm the election chances of his rival, Democratic presidential nominee Hillary Clinton.

  The sources also said that impaneling the grand jury in all likelihood means that the investigation into Russian election meddling will take at least several months.

  Mueller's spokesman, Joshua Stueve, refused to confirm to the Wall Street Journal that the grand jury had been impaneled, while special adviser to the president Ty Cobb said that he did not know if that had been done.

  Cobb said that questions about grand juries typically are kept secret, adding that the White House is in favor of anything that accelerates the conclusion of Mueller's work in a fair way.

  Trump has called Mueller's probe a "witch hunt," adding that the special counsel's appointment in May was very harmful to the US.

Mueller was appointed special counsel in May following the dismissal of FBI Director James Comey. 

  Prior to Mueller's appointment federal prosecutors had already used another grand jury in Alexandria, Virginia, to support an investigation into the Russia contacts of Michael Flynn, Trump's former national security adviser.

  Legal experts consulted by the newspaper said that Mueller's decision to name a grand jury indicates that he is contemplating the possibility of a series of accusations over the long term

  Before he was named special counsel, Mueller had headed the FBI for 12 years under Presidents George W. Bush and Barack Obama until he retired from that post in 2013.