(Adds details on Ukrainian oligarch)
By Nathan Layne and David Alexander
Aug 31 (Reuters) - An American who is the business partner of a Russian national accused by U.S. Special Counsel Robert Mueller of having ties to Russian intelligence pleaded guilty on Friday to unregistered lobbying for a pro-Kremlin political party in Ukraine.
Samuel Patten, 47, also agreed to cooperate with Mueller's probe.
Patten admitted to arranging for a U.S. citizen to act as a straw purchaser to pay $50,000 for four tickets to the inauguration of President Donald Trump on behalf of a Ukrainian oligarch, who reimbursed Patten through a Cypriot account.
The move circumvented U.S. law prohibiting foreigners from providing money to an organization running the inauguration. The tickets were given to the oligarch, another Ukrainian, Patten and the Russian national, according to the charging documents.
While the documents did not name the Russian national, the description matches Konstantin Kilimnik, Patten's business partner, who was indicted along with former Trump campaign chairman Paul Manafort in June for witness tampering in an ongoing criminal case in Washington.
Mueller has said Kilimnik, who once served in the Russian army as a translator, has links to Russian spy agencies. Kilimnik, who in the past has denied such ties, did not respond to a request for comment on Friday.
Patten's plea agreement, which includes cooperating with Mueller, raises the prospect Patten will be called to testify against Manafort, who was found guilty by a Virginia jury last week on bank and tax fraud charges and faces a second trial in Washington next month.
The charge Patten pleaded guilty to - violating the Foreign Agents Registration Act (FARA) by not disclosing lobbying work for Ukrainian politicians - is similar to one of the core allegations against Manafort in his upcoming trial.
Michael Zeldin, a former federal prosecutor, predicted Patten will be a witness in the trial.
"I expect his value as a cooperating witness is he will know how those guys conducted their business relations and can probably shed light on efforts made to evade FARA reporting requirements," he said.
Patten admitted to working with the Russian national to lobby members of Congress and the executive branch on behalf of the pro-Russian Opposition Bloc without disclosing the work to the U.S. government, as required by law.
The charge, which carries a maximum of five years in prison, was brought by the U.S. Attorney for the District of Columbia and the Justice Department's National Security Division, which started investigating Patten after a referral from Mueller.