Burns made the remarks, while testifying alongside with Stuart Levey, US Treasury Undersecretary for terrorism and financial intelligence, before the House Foreign Affairs Committee.

Burns said that the door to serious negotiations is open if Iran is prepared to do so.

European Union foreign policy chief Catherine Ashton will flight to Geneva next week to participate in Iran nuclear talks.

Ashton is due to have talks with Iranian chief nuclear negotiator Saaed Jalili on December 6-7, on behalf of the P5+1, namely the US, Britain, France, Russia and China, plus Germany.

Burns said the sanctions by the UN and US together may inflict a loss of US$50 - 60 billion on Iran in potential energy investments and other sectors.

Levey said the sanctions made it more difficult for Iran to access international financial service.

The US has been imposing sanctions on banks and front companies linked, owned or controlled by Iran, in an effort to isolate it from the international financial system and hence undermine its capacity to develop nuclear program.