The Wall Street Journal newspaper is reporting that Robert Mueller, the US special counsel, has convened a grand jury in Washington, DC to investigate Russia’s interference in the 2016 elections.

The grand jury, which began its work in recent weeks, is a sign that Mueller’s inquiry is gaining steam and that it will probably continue for months, the report says.

Mueller is investigating Kremlin's alleged efforts to influence the 2016 US presidential election and whether President Donald Trump's campaign or associates colluded with Russia as part of that effort.

Grand jury subpoenas have been issued in connection with a meeting on June 2016 between Donald Trump Jr, a Russian lawyer and others, two sources familiar with the matter told Reuters news agency on Thursday.

A grand jury is a group of ordinary citizens who, working behind closed doors, consider evidence and potential criminal wrongdoing that a prosecutor is investigating and decide on whether charges should be brought.

They are powerful investigative tools that allow prosecutors to subpoena documents, put witnesses under oath and seek indictments if there is evidence of a crime.

Legal experts quoted by the Wall Street Journal report said Mueller's decision suggests he believes he will need to subpoena records and take testimony from witnesses.

It also said a grand jury in Washington, DC, is more convenient for Mueller and his 16 attorneys - they work just a few blocks from the US federal courthouse where grand juries meet - than one that is far away in Virginia.

The Kremlin has repeatedly rejected accusations that it tried to influence the US election. Trump has strenuously denied allegations of collusion.

He has also dismissed Mueller's inquiry as a "witch-hunt".

US stocks and the dollar weakened following the news, while US treasury securities gained.

Earlier this week, Reuters reported that Mueller, who was appointed special counsel in May, brought a former US justice department official to join his investigative team.

Greg Andres started on Tuesday, becoming the 16th lawyer on the team.