U.S. Secretary of State Hillary Clinton is treating Iran's letter about restarting nuclear talks with caution, but noted Iran's response was “the one we have been waiting for.”

Clinton and European Union foreign policy chief Catherine Ashton told reporters on Friday that they were happy that Iran was serious about heading back to the negotiation table. But both are curbing their enthusiasm until the talks actually begin.

“We must be assured that if we make a decision to go forward, we see a sustained effort by Iran to come to the table to work until we have reached an outcome that has Iran coming back into compliance with their international obligations,” Clinton said.

“We’re evaluating all of these factors. But I think it’s fair to say ... that we think this is an important step and we welcome the letter.”

The letter was sent to Ashton from Iranian nuclear negotiator Saeed Jalili on Tuesday, and proposed new talks with the P5+1, a group made up of United States, Britain, France, Russia, China and Germany. Formed in 2006, the group was created for nuclear negotiations with Tehran but the two sides rarely agreed on terms for the talks and have had only a few, inconsequential meetings.

“We voice our readiness for dialogue on a spectrum of various issues which can provide ground for constructive and forward looking cooperation,” Jalili said in the note.

Iran has criticized the P5+1 in the past. Just five days ago, Iran said while it was willing to talk, any preconditions established by the P5+1 or United Nations were unwelcome, adding that sanctions had nothing to do with Tehran's change of heart.

“Those who think Iran will change its logical and legitimate approach under pressure and sanctions are mistaken. Iranian people have never compromised on their inalienable rights,” Iran's Ambassador to the United Nations Mohammad Khazaei said in an interview with National Public Radio (NPR).