Robert Mueller, the US special counsel investigating election meddling, 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 Mr Mueller in District of Columbia federal court, included new counts against Mr Trump's former campaign chair Paul Manafort and the operative, Konstantin Kilimnik, for tampering with witnesses about their lobbying for Ukraine.

It was the third time Mr Mueller had added to charges against Mr Manafort since he was indicted in October. Mr 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 the probe, legal experts said. Mr 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 Mr Kilimnik, who in previous court filings was referred to only as "Person A", was named. Mr Mueller has said Mr Kilimnik has links to Russian spy agencies, an allegation Mr Kilimnik denies.