US President Barack Obama said the United States has begun talking with allies about fresh punishment against Iran for defying efforts to halt its nuclear weapons pursuits.

Mr Obama's tough talk came as Iran indicated it would not ship its low-enriched uranium to Russia for processing, the centrepiece of a deal aimed at a peaceful resolution to Iran's contested nuclear programme.

“They have been unable to get to ‘yes’ and so as a consequence we have begun discussions with our international partners about the importance of having consequences,” Mr Obama said in a brief news conference with South Korean president Lee Myung-bak.

Mr Obama did not get more specific about the nature of any new sanctions which would require commitments of international support that are hardly clear yet.

On North Korea, both Mr Obama and Mr Lee signalled impatience and declared fresh, united steps in getting that nation to give up its own nuclear weapons.