At the same time, the official said that the US remains concerned with regard to Iran's nuclear weapons programme.
"The IAEA has provided the United States with a copy of Iran's letter and has requested our views. We understand that France and Russia, as parties to the original IAEA proposal, have also received copies of the letter. We are consulting with our partners and expect to respond to the IAEA very soon," State Department spokesman P J Crowley told reporters.
The formal US response is expected to be later this week, he said.
"But our concerns about Iran go beyond just issues regarding the Tehran research reactor. Both the unwillingness of Iran to engage seriously with the international community, and in particular with the P-5+1, in addition to the ongoing enrichment of nuclear material to 20 percent and Iran's continued defiance of several UN Security Council resolutions," he said.
Crowley said the US is going to consult with France and Russia before responding formally to the letter.
"As we indicated last week, we do appreciate the efforts of Brazil and Turkey in trying to push Iran to be more forthcoming to address the concerns the international community has. It's unclear if the letter truly offers anything new, but we will study it closely and will respond formally through the IAEA in the next few days," he said in response to a question.
The State Department spokesman, however, denied reports that the US is undermining the efforts of Brazil and Turkey in this regard.
"We work with Brazil on a wide range of issues, regional and global. Brazil, like Turkey, are emerging global powers and are stepping up in terms of wanting to play and actual playing a significant role in tackling the challenging issues that we face around the world. Climate change is one. But nonproliferation is another," he said.
"We do appreciate the earnestness with which Brazil and Turkey have tried to mediate this, but this is really about Iran. It's not about Turkey; it's not about Brazil. It is about whether what is on paper today represents a real change in the attitude of Iran to address the concerns of the international community has. We will study that closely, we'll consult, and then respond formally later this week," Crowley said.
Iran has been made clear that if it chooses to develop a nuclear weapon, that will kick off an arms race in the Middle East that will challenge the foundation of the NPT.
"As we've stressed, Iran and many other countries have the right to develop civilian nuclear energy, but with that right comes responsibilities.
And Iran, by insisting that it will continue to enrich uranium to 20 percent, is in violation of a number of UN Security Council resolutions," he said.
"That's why we seek to engage Iran directly on these issues. It's why we called for the meeting last October 1st. And we will continue to look to see if Iran is prepared to engage the international community seriously. I think over the weekend, Catherine Ashton (of the European Union) reiterated the willingness of the P-5+1 to meet anytime, anywhere, provided that Iran is willing to engage seriously on these nuclear question," Crowley said.
