Special counsel Robert Mueller has impaneled a grand jury to investigate Russia's interference with the 2016 presidential election, The Wall Street Journal reported Thursday -- a step toward possible criminal indictments.
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 in the Republican's favor -- is gathering pace.
The establishment of a grand jury will allow Mueller -- a former FBI director -- to subpoena documents and get sworn testimony. It could lead to criminal indictments.
"It's a significant escalation of the process," national security attorney Bradley Moss told AFP.
"You don't impanel a grand jury unless your investigation has discovered enough evidence that you feel reflects a violation of at least one, if not more, criminal provisions," he said.
"If you secure an indictment, your next step is to arrest the defendant."
Presidential lawyer Ty Cobb said he was not aware that a grand jury had been convened.
"Grand jury matters are typically secret," Cobb said, adding that "the White House favors anything that accelerates the conclusion of his work fairly."
"The White House is committed to fully cooperating with Mr Mueller."
White House spokeswoman Sarah Huckabee Sanders said the president was not likely the subject of an investigation.
"Former FBI director Jim Comey said three times the president is not under investigation and we have no reason to believe that has changed," she said.
Justice Department spokeswoman Sarah Isgur Flores declined to comment on the report.
Trump has repeatedly denied allegations of collusion, saying he is the victim of a political "witch hunt" and "fake news."
But the White House has been forced to acknowledge that Trump's eldest son, Donald Jr, his son-in-law Jared Kushner and his then campaign advisor Paul Manafort did meet a Kremlin-connected lawyer to get dirt on Hillary Clinton.
Mueller is also said to be investigating financial records of Trump associates unrelated to Russia or the election, CNN reported.
Trump has publicly warned Mueller that his financial dealings should be out of bounds and investigating them would cross a red line.
The revelation will only fuel speculation that Trump may try to curb the investigation by firing Mueller.
Two US senators introduced a bipartisan bill Thursday to pre-empt that move by insulating Mueller.
The legislation, sponsored by Democrat Chris Coons and Republican Thom Tillis, would bar a president from directly firing the special counsel without a judicial review.
Under the bill, Mueller would be allowed to challenge his removal in court in the event he is fired without good cause.
"A back-end judicial review process to prevent unmerited removals of special counsels not only helps to ensure their investigatory independence, but also reaffirms our nation's system of check and balances," Tillis said in a statement.
Coons added: "Ensuring that the special counsel cannot be removed improperly is critical to the integrity of his investigation."