Iran says it is developing nuclear power for civilian purposes
The six powers co-ordinating policy with Iran over its nuclear ambitions have moved to invite the Iranians for fresh talks on a diplomatic solution.
The US, Russia, China, France, Germany and Britain announced they would ask EU foreign policy chief Javier Solana to approach Iran with the offer.
Signalling a policy shift, US Secretary of State Hillary Clinton said the US would be a "full participant" in talks.
Iran denies it is seeking to build a nuclear weapon.
The six countries, made up of the five permanent members of the UN Security Council plus Germany, released a statement calling for co-operation from Iran.
They said they "strongly urge Iran to take advantage of this opportunity to engage seriously with all of us in a spirit of mutual respect".
The six said they would ask Mr Solana to "extend an invitation to the Iranian government" to meet their representatives "so that together we may find a diplomatic solution to this critical issue".
No time frame was given for the proposed offer of talks.
The group's statement also reaffirmed its members commitment to a "dual track" strategy, promising economic and political assistance if Iran agreed to international demands to halt enrichment operations, but tighter sanctions if they do not.
Mrs Clinton said careful engagement with Iran made sense
The BBC's Kim Ghattas in Washington says the US decision to be fully involved in the talks marks a clear departure from the policies of the previous White House administration.
In a video message last month, President Barack Obama offered "a new beginning" to the Iranian people and leaders.
"My administration is now committed to diplomacy that addresses the full range of issues before us," Mr Obama said.
When Undersecretary of State William Burns attended an Iranian meeting last year, his participation was restricted, but this position will now change, says our correspondent.
Commenting on the policy shift, Mrs Clinton said: "Obviously we believe that pursuing very careful engagement on a range of issues that affect our interests and the interests of the world with Iran makes sense."
"There is nothing more important than trying to convince Iran to cease its efforts to obtain a nuclear weapon," she told reporters.
The talks are based on international concern that Iran's nuclear programme is a cover to build atomic weapons.
Enriched uranium can be used in nuclear power plants, but can also be used to make the weapons.
But Tehran has always insisted it is developing nuclear power for civilian purposes.