An Iranian official said today that Iran will not suspend uranium enrichment as demanded in a nuclear package backed by six world powers.
He said Tehran would formally respond to the package tomorrow, a deadline Iran had earlier set itself.
"Considering the technical advancement of Iranian scientists, the suspension of uranium enrichment is not possible any more," the deputy head of Iran's Atomic Energy Organisation, Mohammad Saeedi, was quoted as saying.
Iranian officials have previously insisted Iran would not stop enrichment, a process that has military and civilian uses, but world powers were hoping to sway that decision.
The West accuses Iran of seeking to develop atomic weapons under cover of a civilian nuclear programme. The West says Iran must halt enrichment before any talks can start.
Tehran denies it seeks to make bombs, saying it only wants to enrich uranium to make fuel for nuclear power plants.
In addition to concerns about enrichment, Western nations are also worried about a heavy-water nuclear reactor being built at Arak, 120 miles southwest of Tehran. The plant's plutonium by-product can be used to make atomic warheads.
Spent fuel can be processed to extract weapons-grade plutonium. The plutonium can also be mixed with enriched uranium to produce fuel for a special type of nuclear reactor.