Iran says it will breach the limit on its stockpile of enriched uranium that was set under a 2015 nuclear deal with world powers.
Iran's atomic energy agency says it has quadrupled production of the material, which is used to make reactor fuel and potentially nuclear weapons.
But it added there was "still time" for European countries to act by protecting Iran from reinstated US sanctions.
The UK, France and Germany have warned Iran not to violate the deal.
A US National Security Council spokesman as saying the enrichment plans were "nuclear blackmail" and should be met with "increased international pressure".
Simon Marks is our correspondent in Washington DC. He talks to Susie Ferguson.
