Iran's president has said that there is no legal basis for more sanctions against Tehran after a US intelligence report concluded that it stopped trying to build a nuclear bomb four years ago.
Mahmoud Ahmadinejad's comments on Tuesday came as the US, Russia, China, France, Britain and Germany met to discuss a new UN resolution punishing Iran.
"We think that any such measure has lost its legal basis," Ahmadinejad said.
"We had two rounds of sanctions and what happened? Nothing," he said in a two-hour news conference. "Now is the time for them to change their behaviour."
Washington is continuing to push for more sanctions despite the findings of the intelligence estimate.
The UN Security Council could vote on the new resolution within weeks.
On Tuesday, George Bush, the US president, repeated his belief that Iran was still "dangerous", and would be more so if it began enriching uranium.
"We believe Iran had a secret military weapons programme, and Iran must explain to the world why they had such a programme, he said after a meeting with Giorgio Napolitano, Italy's president.
Necessary insight
Earlier, Greg Schulte, the US ambassador to the International Atomic Energy Agency (IAEA), told Al Jazeera that Tehran remained a concern because it had failed to disclose information to the UN watchdog.
"It [the intelligence assessment] told me that Iran could restart that programme if it wanted to and we have to be concerned about that because Iran is not giving the IAEA the necessary insight into their nuclear activities," he said.
The two previous rounds of sanctions were enforced against Tehran after it refused to halt sensitive uranium enrichment work.
The process can provide material for use either in a civilian nuclear power programme or in an atomic weapon.
Iran insists that its nuclear programme is entirely peaceful and Ahmadinejad disputed the contention that Iran ever had a weapons programme.
"We do not say that in the report there is no problem and there is no imprecision or error. We welcomed the report favourably as a whole and it is a step forward," he said on Tuesday.
The report said that US allegations about Iran's atomic goals had been overblown for at least two years, but it added that it could have the capability to make a nuclear weapon by 2015.