"Given that Mueller inherited an investigation that began months ago, it would suggest that he has uncovered information pointing in the direction of criminal charges. But against whom is the real question."

A lawyer for Trump, Jay Sekulow, appeared to downplay the significance of a grand jury, telling Fox News: "This is not an unusual move."

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

It was not immediately clear to whom subpoenas were issued and the sources did not elaborate.

Some lawyers said it would put pressure on potential witnesses to cooperate with Mueller's investigation.

"When someone gets a subpoena to testify, that can drive home the seriousness of the investigation," said David Sklansky,a professor at Stanford Law School and a former federal prosecutor.

In 2005, a grand jury convened by Special Counsel Patrick Fitzgerald returned an indictment of Lewis "Scooter" Libby, a top aide to then-Vice President Dick Cheney.

"A special counsel can bring an indictment and it has happened before," said Renato Mariotti, a partner at the law firm Thompson Coburn and a former federal prosecutor.

Reuters