(Reuters) - U.S. Special Counsel Robert Mueller filed charges on Friday against a political operative with alleged ties to Russian intelligence, bringing his probe into possible collusion by the Trump presidential campaign a step closer to the Kremlin’s door.

The indictment, filed by Mueller in District of Columbia federal court, included new counts against Trump’s former campaign chair Paul Manafort and the operative, Manafort aide Konstantin Kilimnik, for tampering with witnesses about their lobbying for Ukraine.

It was the third time Mueller had added to charges against Manafort since he was indicted in October. Manafort has pleaded not guilty to a raft of charges, from money-laundering, to failing to register as a foreign agent, to bank and tax fraud.

The additional charges could add to pressure on Manafort to cut a deal and cooperate with Mueller’s probe, legal experts said. Manafort has longstanding ties to a pro-Russian political party in Ukraine and a Russian oligarch close to the Kremlin.

But more significantly Friday’s indictment marked the first time that Kilimnik, who in previous court filings was referred to only as “Person A”, was named. Mueller has said Kilimnik has links to Russian spy agencies, an allegation Kilimnik denies.

Kilimnik did not immediately respond to a request for comment. Manafort and his lawyers have disputed the charges against him.

The disclosure could alter the way the public perceives Mueller’s investigation, said former federal prosecutor Renato Mariotti. Trump has repeatedly denounced the probe as a “witch hunt” and denied any collusion took place.

“This indictment could be very important from a political perspective,” said Mariotti, a Democrat who is running for Illinois attorney general. “You now have the former chairman of the Trump campaign charged with conspiring with a suspected Russian intelligence operative. That’s quite astounding.”

In a court filing earlier this week, Mueller asked the judge overseeing the case in the District of Columbia to revoke or revise an order releasing Manafort ahead of trial due to the allegations of witness tampering.

FILE PHOTO Paul Manafort, former campaign manager for U.S. President Donald Trump, departs after a hearing at U.S. District Court in Washington, DC, U.S., April 19, 2018. REUTERS/Brian Snyder/File Photo

Mueller has accused Manafort of attempting to call, text and send encrypted messages in February to two people from “The Hapsburg Group,” a political discussion group he allegedly worked with to promote the interests of Ukraine, in an effort to influence their testimony or conceal evidence in the case.

Manafort’s lawyers, in a court filing on Friday, asked the judge to reject Mueller’s request, saying “the text messages cited by the special counsel do not establish any witness tampering.”

They said Manafort’s “limited communications cannot be fairly read, either factually or legally, to reflect an intent to corruptly influence a trial witness.”

Mueller’s office said the government had “developed substantial evidence showing that the Hapsburg group performed a variety of lobbying tasks in the United States,” which included disseminating ghostwritten articles in the American media and arranging meetings with U.S. officials and politicians.

Sources familiar with the group’s activities said that among its major figures were Alfred Gusenbauer, a former Austrian chancellor, and Romano Prodi, a former Italian prime minister.

In Friday’s indictment, Mueller accused Kilimnik of taking part in the covert lobbying scheme and of trying to influence potential witnesses.

Two U.S. intelligence officials on Friday said that Kilimnik, who was educated in part as a linguist and served in the Russian army as a translator, is believed to have at least informal ties to Russian intelligence.

They said Kilimnik may have reported to Russian intelligence officers in Moscow or Kiev or both on his work at the International Republican Institute office in Moscow where he was employed for a decade to 2005 and later on his work for Manafort in Ukraine.

In addition, one of the officials said, Kilimnik’s reported trips to the U.S. in May and August 2016 during Trump’s campaign have “attracted attention” as to whether Manafort and Kilimnik tried to capitalize on Manafort’s campaign role by offering Russian oligarch Oleg Deripaska private briefings.

Deripaska has denied accepting the offer or receiving any such briefings.