France called the move a "grave error", and the UK said the world was running out of patience with Iran. The EU and US immediately also condemned this latest move.

The renewal of research on Tuesday at the Natanz site suggests that all of Iran's nuclear activities may have been revived.

Iran resumed uranium conversion, an earlier stage in the nuclear fuel cycle, at its plant in Isfahan when negotiations with the EU had broken down in August.

Then in September, the International Atomic Energy Agency's board called on Iran to cease all nuclear fuel work, and threatened to refer Tehran to the Security Council.

But Tehran has always said it has the right under the nuclear Non-Proliferation Treaty to research nuclear energy for peaceful purposes and this is what the country says it is doing.