"Iran is trying to resolve these ambiguities and I think this letter is part of that process," Marandi said.
The "freeze-for-freeze" proposal was an offer made by the UK, China, France, Russia, the US and Germany to refrain from steps to impose more sanctions if Iran freezes expansion of its nuclear work.
The proposal was an initial step in getting talks going on a broader solution, according to the six countries.
Mansour Farhang, Iran's former ambassador to the UN, told Al Jazeera that "enrichment of uranium is the only card Iran has to play," in its dealings with Western powers.
"Giving up that card means it has to engage in negotiations without having any ability to pursue its interests."
For a solution to the stalemate between Iran and the Western powers, "non-enrichment has to be the goal of negotiations, not a precondition", he said.
Gonzalo Gallegos, a US state department spokesman, said: "If we are not going to receive a clear response, a clear message from them, we are going to have no choice but to pursue additional measures."
|Iran says it is ready to provide
a clear response [EPA]
An extract of the letter, obtained by Reuters, said: "Iran is ready to provide a 'clear response' to your proposal at the earliest possibility, while simultaneously expecting to receive your 'clear response' to our questions and ambiguities as well."
Gallegos said negotiators from the six countries have scheduled a conference call on Wednesday to discuss the issue.
"First we will see what happens with the rest of the day that is left in Europe, and then they will have their call tomorrow. We will see where we go from there."
Elahe Mohtasham, a nuclear specialist from the Foreign Policy Centre in London, told Al Jazeera that Iran's position "hasn't really changed as far as any suspension of its nuclear centrifuge activity is concerned".
"The best bet - in terms of the interests of the West - is to start negotiatons without any precondiditions ... and then in the course of negotations maybe we will be able to get more concessions from Iran," she said.
"Sanctions haven't worked and are very unlikely to work. Any further sanctions would really increase the price of oil and wouldn't benefit the world economy."
The UK, France and the US had warned that Iran would face a fourth set of UN Security Council sanctions if it failed to produce a response to the package by Tuesday.
The new deadline was set after Iran ignored a previous demand to respond by last weekend to the proposed package.
Tensions were again heightened on Monday, after Iran said it had successfully test-fired an anti-ship missile with a range of 300km that would allow it to close the Strait of Hormuz between Iran and Oman.
General Mohammad Ali Jafari, the commander of Iran's Revolutionary Guards, said: "No enemy vessels would be able to escape it."
Tehran has steadfastly refused to suspend its uranium-enrichment activities, which it says are aimed only at producing fuel for nuclear power production.
Western powers fear the programme is a cover for developing nuclear weapons.