U.S. Secretary of State Hillary Clinton says if major powers resume talks with Iran about its nuclear program, the discussion would have to be a “sustained effort that can produce results.”

Speaking at the State Department Friday with European Union foreign policy chief Catherine Ashton, Clinton said the U.S., EU and their partner countries are reviewing Iran's proposal to resume talks.

“The international community has been looking to Iran to demonstrate it is prepared to come to the table in a serious and constructive way. We have been reviewing Iran's proposal to resume talks on its nuclear activities and consulting closely between us and with our other P5+1 partners. This response from the Iranian government is one we've been waiting for and if we do proceed, it will have to be a sustained effort that can produce results.”

Ashton said it is good to see that the letter from Iranian nuclear negotiator Saeed Jalili has arrived and that there is a possibility that Iran may be ready to start talks. She said she is cautious and optimistic at the same time.

The United States and the EU have been increasing sanctions against Iran because of concerns it is trying to develop nuclear weapons. Iran insists its nuclear program is peaceful.

A major international banking hub, the Society for Worldwide Interbank Financial Telecommunication, or SWIFT, said Friday it is prepared to discontinue services to Iranian banks targeted by EU sanctions. It said it is ready to act as soon as it has clarity on EU legislation currently being drafted. Washington has been urging the EU to block Iranian banks access to the cooperative.