Switzerland, a major physical oil trading hub and base for Iranian state-owned oil trading firm Naftiran Intertrade Company, extended an Iran blacklist in April but postponed a decision on the EU ban until a "later date".

It also exempted Iran's central bank from its asset freeze.

"We expressed our disappointment. We would like them to do it [follow the EU on Iran sanctions]," Donald S. Beyer told a news conference in Geneva on Wednesday.

The EU embargo bans all new oil contracts with Iran and is due to take full effect for existing contracts from 1 July.

Beyer added that Switzerland had cited its traditional role as a go-between for Washington and Tehran and its history of neutrality as reasons for not following the EU on Iran sanctions.

Switzerland has represented U.S. interests in Iran since 1980 following the Islamic revolution in 1979. It is not a member of the EU and has no legal obligation to follow EU sanctions, which are designed to pressure Tehran over its disputed nuclear programme.

Switzerland's body for enforcing sanctions, the Swiss Secretariat for Economics, said on Thursday the Federal Council had not yet indicated when it would make a final decision on the Iranian sanctions.

"My reading of the Swiss leadership is they very much do not want Switzerland to become an outlet for evading sanctions," Beyer said.

"I don't think that's what they want or what they would tolerate."

It is unclear how much of Iran's oil exports, estimated at below 2 million barrels per day, are currently traded or financed via Switzerland, where trading of a third of total global oil by volume takes place.

Some major commodities traders such as Zug-based Glencore say they have already voluntarily halted all oil trade with Iran.