US Commerce Secretary Wilbur Ross has business ties to a shipping firm linked to Vladimir Putin's inner circle, leaked documents showed on Sunday.Ross, a billionaire investor, holds a 31 per cent stake in Navigator Holdings through a complex web of offshore investments detailed in leaked documents from a law firm examined by nearly 100 news organisations as part of an international collaboration.The 79-year-old reduced his stake when he took public office, according to public filings.Navigator Holdings runs a lucrative partnership with Russian energy giant Sibur, which is partially owned by Putin's son-in-law Kirill Shamalov and Gennady Timchenko, the Russian president's friend and business partner who is subject to US sanctions.The US imposed sanctions on Russian entities and individuals following its annexation of Crimea and aggression in Ukraine.While there is no evidence Ross acted illegally, the documents -- part of millions of documents dubbed the Paradise Papers obtained from offshore law firm Appleby -- raise questions about whether his investments undermine the US measures.Ross's private equity firm has been the biggest shareholder in Navigator. His personal share of the firm's stake was reduced when he took office in February, but the commerce chief's investment is still valued at between $2 million to $10 million, according to his security filings and government ethics disclosure.The New York Times reported that Ross's stake in Navigator has been held by companies in the Cayman Islands. His wealth, estimated to exceed $2 billion, is said to be tied to similar arrangements in various tax havens like the Cayman Islands."Secretary Ross was not involved with Navigator's decision to engage in business with Sibur, a publicly traded company, which was not under sanction at the time and is not currently," said James Rockas, a Commerce Department spokesman."Moreover, Secretary Ross has never met the Sibur shareholders referenced in this story and, until now, did not know of their relationship."He added that Ross recuses himself from matters focused on transoceanic shipping vessels, "but has been supportive of the administration's sanctions against Russian and other entities."The investments emerged as part of the Paradise Papers leak by the US-based International Consortium of Investigative Journalists, which was behind the 2015 Panama Papers release.The latest haul contains 13.4 million documents mainly from Appleby, an offshore law firm with offices in Bermuda and beyond.The files were first obtained by the German newspaper Suddeutsche Zeitung, and shared with the ICIJ and partner media outlets.