Iran delivered a letter to world powers on Tuesday but gave no concrete reply to a demand to freeze its nuclear activity.
Iran delivered the letter to European Union foreign policy chief Javier Solana in response to an offer in June by Britain, China, France, Germany, Russia and the United States to refrain from moving to impose more UN penalties if Iran froze expansion of its nuclear work.
They fear Iran wants to build nuclear weapons. Iran insists it merely wants to generate electricity. Iran is the world's fourth-largest oil producer.
The UN Security Council has imposed three rounds of penalties on Iran since 2006. The US also maintains its own sanctions against Iran.