U.S. Undersecretary of State Nicholas Burns will take part in today's meeting in London with officials from the three EU countries spearheading efforts to persuade Tehran to give up all uranium-enrichment activities.
Next week, the board of the International Atomic Energy Agency meets to discuss whether to refer Iran to the UN Security Council for possible sanctions.
IAEA inspectors are reported to have witnessed a new round of uranium conversion at the Isfahan facility in Central Iran.
Iran rejects American charges it is using its nuclear program to develop nuclear weapons.