Mohamed ElBaradei, the head of the UN's atomic agency, has said Iran could develop a nuclear weapon in three to eight years if it so chooses.
Mr ElBaradei has faced criticism from Europe and the US
Mr ElBaradei said he wanted to prevent Iran from enriching uranium on an industrial scale and to use talks to ease tensions over its nuclear work.
The US says it will press for new sanctions on Iran over its continued failure to suspend uranium enrichment.
Iran denies US claims that its nuclear programme has a military aspect.
The UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), said in a report on Wednesday that Tehran was stepping up enrichment and obstructing inspections.
The US described the report as "a laundry list of Iran's continued defiance of the international community".
But Iranian President Mahmoud Ahmadinejad said Iran would not be deterred from pursuing its nuclear goals.
"The enemies want us to surrender so that Iran won't have anything to say in the world," Mr Ahmadinejad said on Thursday.
"With the backing of the Iranian nation we are not afraid of the enemies'... psychological warfare and with God's help we have come close to our ultimate goals," he said.
Mr ElBaradei addressed a joint US-Russian conference on nuclear proliferation in Luxembourg on Thursday.
"Even if Iran wanted to go for a nuclear weapon, it would not be before the end of this decade or sometime in the middle of the next decade," he said, adding that this estimate echoed the view of US intelligence agency, the CIA.
The US and its European allies have rejected Mr ElBaradei's suggestion that Iran be allowed to keep part of its enrichment programme.
In March, the UN Security Council imposed a second round of sanctions on Iran for failing to suspend uranium enrichment, prompting Tehran to limit its co-operation with the IAEA.
The BBC's diplomatic correspondent, Jonathan Marcus, says Washington may have some difficulty in persuading all members of the UN Security Council to impose further sanctions.
IAEA inspectors still regularly visit Iran's atomic facilities under the nuclear Non-Proliferation Treaty.
Iran has denied hindering the IAEA and says it will "continue to co-operate".