ZURICH, April 20 (Reuters) - Switzerland will maintain tight non-disclosure rules as part of its Russian sanctions package and keep conversations with trading houses on their purchases of Russian crude confidential, the government agency in charge of sanctions said on Wednesday.

The State Secretariat for Economic Affairs (SECO) had said on Tuesday it would decide case by case whether to curtail traders' purchases from Russia's state-controlled companies under European Union sanctions Bern has adopted that limit commodity deals to those deemed "strictly necessary" from mid-May.

Asked by Reuters how SECO planned to communicate such decisions, a spokesperson said: "We do not comment publicly on individual transactions. These are not accessible."

Switzerland is a major hub for trading Russian commodities.

Major global trading houses are planning to reduce crude and fuel purchases from state-controlled Russian companies such as Rosneft and Gazpromneft as early as May 15, sources told Reuters last week, to avoid falling foul of European Union sanctions on Russia.

The EU has not imposed a ban on imports of Russian oil in response to Russia's invasion of Ukraine, because some countries such as Germany are heavily dependent on Russian oil and do not have the infrastructure in place to swap to alternatives.

Trading companies are, however, winding down purchases from Russian energy group Rosneft as they seek to comply with language in existing EU sanctions that were intended to limit Russia's access to the international financial system, sources told Reuters last week.

The wording of EU sanctions exempts oil purchases from Rosneft or Gazpromneft, which are listed in the legislation, deemed as "strictly necessary" to ensure Europe's energy security. (Reporting by Michael Shields; Editing by Toby Chopra)