The brief letter, which offered no specific proposals in the nuclear dispute, may not convince Western states that Iran is ready to enter the kind of substantive negotiations tackling its nuclear work they say is needed to resolve the longstanding row.
"We voice our readiness for dialogue on a spectrum of various issues which can provide ground for constructive and forward-looking cooperation," said the letter, dated February 14, from Iranian chief nuclear negotiator Saeed Jalili.
The United States and its allies suspect Iran is seeking to develop nuclear weapons capability. Iran denies this, saying it is enriching uranium only for electricity to meet demands of its rapidly growing population.
Jalili's letter was a response to one from Ashton in October, in which she said the big powers could meet with Iran within weeks if it was ready to "engage seriously in meaningful discussions" and address concerns about its nuclear work.
Ashton handles the Iran file on behalf of six world powers - the United States, Russia, China, Germany, France and Britain.
Negotiations have been frozen since a fruitless meeting in Istanbul in January 2011.
Tension has soared in recent months after the UN nuclear watchdog in November issued a report on Iran detailing indications that it has worked on designing a nuclear weapon.
Jalili said he welcomed a statement by Ashton on respecting Iran's right to the peaceful use of nuclear energy.
"No doubt by committing to this approach, our talks for cooperation based on step-by-step principles and reciprocity on Iran's nuclear issue could be commenced," the English-language letter said.
"(A) constructive and positive attitude towards Islamic Republic of Iran's new initiatives in this round of talks could open positive perspective for our negotiation."
"Therefore...I propose to resume our talks in order to take fundamental steps for sustainable cooperation in the earliest possibility in a mutually agreed venue and time," Jalili said.
He did not specify what initiatives he was referring to.
In previous talks, Iran has refused to discuss measures that would guarantee its nuclear program is peaceful and transparent for UN inspectors, instead floating vague proposals for international cooperation on security and trade.
A spokeswoman for Ashton confirmed receipt of the letter on Wednesday, saying she was evaluating it and would consult with the six powers on a response.
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