Special Counsel Robert Mueller leaves the U.S. Capitol Building after meeting with members of the Senate Judiciary Committee in Washington, U.S., June 21, 2017. REUTERS/Aaron P. Bernstein/File Photo

(Reuters) - The term of the grand jury being used by U.S. Special Counsel Robert Mueller in his investigation of possible collusion between Russia and President Donald Trump’s 2016 election campaign has been extended, an aide to the judge overseeing it said on Friday.

The extension is a sign that Mueller is not done presenting evidence before the grand jury in his investigation of U.S. allegations of Russian interference in the election and any possible coordination between Moscow and Trump’s campaign.

The grand jury was impaneled by the U.S. District Court in Washington in July 2017 for an 18-month term, the limit under federal rules. The term can be extended if the court determines it to be in the public interest to do so.

“The Chief Judge has confirmed that the term of Grand Jury 17-01 has been extended,” Lisa Klem, special assistant to Chief Judge Beryl Howell said in a statement.

Howell did not confirm any length of the extension, Klem said.

A spokesman for Mueller declined to comment.

A number of Trump’s allies, including his lawyer Rudy Giuliani, have repeatedly called on Mueller to wrap up his investigation.

Trump has called the investigation a “witch hunt” and has denied collusion with Moscow. Russia has denied meddling in the election, contrary to the conclusion of U.S. intelligence agencies that have said Moscow used hacking and propaganda to try to sow discord in the United States and boost Republican Trump’s chances against Democratic candidate Hillary Clinton.

Mueller’s investigation and other inquiries have clouded Trump’s two years in office. Mueller has secured more than 30 indictments and guilty pleas and has spawned at least four federal probes.