Iran's foreign ministry said the letter aimed to explain the country's position on the negotiations, saying that Iran had been resisting "illegal" and "humiliating" demands from its counterparts in the talks.
"Iran proposed practical, equal and realistic offers based on mutual respect," the minister said, adding that his country showed determination to reach a fair and reasonable agreement.
The U.S., U.K., France, China, Russia plus Germany comprise the P5+1 group of world powers who are negotiating with Iran over its uranium enrichment, nuclear cooperation and heavy-water reactor plants.
The negotiations plan to reach a joint agreement, which will lead to the removal of economic sanctions and the implementation of a peaceful nuclear program.
The latest round of the nuclear talks ended in Vienna on Nov. 24 without a permanent agreement. Both sides, however, agreed to extend the talks until July 1, 2015.
Zarif said Iran neither sought nor needed nuclear weapons which he said were not effective in providing external security and internal peace. "These weapons are detrimental to Iran's security," he said.
Western countries agreed to ease some of its international sanctions on Nov. 24, 2013 in exchange for Iran limiting certain aspects of its nuclear activities.