But prosecutors say they are concerned that Firtash — who is wanted on racketeering charges and has ties to President Donald Trump's former campaign manager Paul Manafort — will jump on a private jet to Russia if a Chicago judge rules against him before he is handed over to U.S. authorities, according to Chicago Tribune.

They revealed in court for the first time that they wiretapped telephone conversations in which Firtash allegedly discussed a co-defendant's trip to Chicago to meet with Boeing executives.

Firtash, who has friends in Russian President Vladimir Putin's Kremlin, has been fighting extradition from Austria since his high profile 2014 arrest in Vienna. Accused of masterminding an international titanium-mining racket involving Boeing, he claims he was targeted by the Obama administration as punishment for Putin's annexation of Crimea.

Read alsoAustria refuses to hand Firtash over to Spain - mediaHis lawyer, former U.S. Attorney Dan Webb, on Monday told U.S. District Court Judge Rebecca Pallmeyer that Firtash has never been to the U.S., and the crimes of which he is accused all happened in India and had no impact on the U.S.

Webb said he feared Firtash's extradition is imminent, and urged Pallmeyer to throw out the case before Firtash can be put on a Chicago-bound plane.

But prosecutor Amarjeet Bhachu told the judge that the wiretapped conversations linked Firtash to Chicago, and that a bribery scheme also used U.S. banks and cellphones.

He warned the judge that if she ruled against Firtash before he is in U.S. custody, he may "hop on a private plane and head over to Moscow, or to some other country where we can't extradite him."

In a joke about Firtash's enormous wealth (he posted a $174 million bond in Austria), Bhachu added, "We will pay for his travel here."

Read alsoUkrenergo denies possible electricity supply to CrimeaU.S. interest in the case in recent months has focused on Firtash's links to Manafort, with whom he discussed a New York real estate deal in 2008. Though Manafort is not named in the Chicago case, his home has been raided as part of special prosecutor Robert Mueller's probe of the Trump campaign's alleged ties to Russia.