An EU source in Brussels could not confirm the report.
The six world powers have offered to refrain from steps to impose more U.N. sanctions if Iran freezes expansion of its nuclear work -- an initial step in getting talks going on a broader resolution to the stand-off.
Western officials, who fear Iran wants to build a nuclear bomb despite its denials, said they had been told Tehran would give a written reply on Tuesday.
"Iran's written response to the six countries involved in the nuclear negotiations was handed to officials at the European Union by Iran's ambassador to Brussels," Fars News Agency reported, without giving any further details.
The freeze idea is aimed at getting preliminary discussions going before starting full negotiations on a package of nuclear, trade and other incentives. But those formal talks will not begin until Iran suspends uranium enrichment.
Enrichment is the part of Iran's programme that most worries the West because it can have both civilian and military uses.
Iran, the world's fourth largest oil producer, insists it is only seeking to master technology to make electricity, and has repeatedly refused to halt its atomic work.
Washington and its Western allies said on Monday that, if Iran's response was not positive, the next step would be to expand U.N. sanctions. The U.N. Security Council has imposed three rounds of penalties on Iran since 2006.
Russia and China, two members of the sextet, have been reluctant to impose sanctions in the past but have, in the end, voted for all three sanctions resolutions after initial drafts were watered down.
The others in the sextet are the United States, Britain, France and Germany.
(Source: Reuters India)