Special Counsel Robert Mueller has impaneled a grand jury to investigate Russia’s meddling in the 2016 election, The Wall Street Journal reported Thursday.

The move is seen as a sign the investigation into election interference and possible collusion with the Trump campaign is heating up and entering a new phase. Reuters also reported that grand jury subpoenas have been issued in connection with the meeting Donald Trump Jr. had with a Russian lawyer and others last year.

Russia has denied having a hand in the U.S. presidential election. President Trump, too, has strongly denied allegations of collusion and has frequently called the investigation a “witch hunt.”

Grand juries allow prosecutors to subpoena documents and get witness testimony on the record. They also can seek indictments.

Ty Cobb, special counsel to the president, told Fox News on Thursday he wasn’t aware that Mueller had impaneled a new grand jury but said, "We favor anything that brings this investigation to a swift conclusion."

Cobb also told reporters that, "Grand jury matters are typically secret," and the White House “is committed to fully cooperating with Mr. Mueller."

Trump attorney Jay Sekulow downplayed the significance of the grand jury, calling it "a standard operating procedure when you've got a situation like this."

“We have no reason to believe President Trump is under investigation,” Trump attorney John Dowd told Fox News.

White House Press Secretary Sarah Huckabee Sanders noted that former FBI Director James Comey repeatedly said Trump was not under investigation earlier this year.

Mueller, who now has a team of 16 attorneys, was brought on as special counsel in May. Prior to his involvement, federal prosecutors reportedly had been using another grand jury, in Alexandria, Va., to help with their criminal investigation into former national security adviser Michael Flynn.

The Flynn case focuses on his work in the private sector on behalf of foreign interests.

Asked Thursday about the Journal report, the special counsel's office had no comment.

Trump and his allies have in recent weeks openly criticized Mueller, with one Republican lawmaker even calling for him to step aside.

Other lawmakers have responded with proposals that could serve to shield Mueller from being fired.

Trump also has fumed about Attorney General Jeff Sessions' decision to recuse himself from the Russia case, which cleared the way for Mueller's eventual appointment -- though Sessions has recently been told by the White House his job is considered safe.

Thomas Zeno, a former federal prosecutor, told the Journal that impaneling a grand jury is “confirmation that this is a very vigorous investigation going on.”

Zeno cautioned the step does not automatically mean Mueller will bring charges but added “it shows he is very serious. He wouldn’t do this if [the investigation] were winding down.”

Fox News' John Roberts contributed to this report.