Iran has announced it will breach on 27 June the limit on its stockpile of enriched uranium that was set under a 2015 nuclear deal with world powers.
Its atomic energy agency said it had 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.
They have previously said they will have no choice but to reimpose their own sanctions, which were lifted in return for limits on the Iranian nuclear programme.
This latest development comes at a time of high tension in the Middle East, with the US accusing Iran of being behind suspected attacks that left two oil tankers ablaze in the Gulf of Oman on Thursday. Iran has denied any involvement.
How significant is this announcement?
In early May, Iran signalled its intention to break out from some of the constraints imposed by the nuclear deal. On Monday, it repeated that threat, adding that the rate of enrichment was being stepped up and that it expected to break the limit on the amount of low-enriched uranium it can hold before the end of this month.
But since its first announcement, the context has changed dramatically with two sets of attacks on oil tankers in the Gulf - both attributed by the US to Iran.
Many fear that Washington and Tehran are on the verge of a military clash and that miscalculation could tip them over the edge.
Iran's further threat today to increase its level of enrichment to a stage that would bring it much closer to "weapons-grade" material adds another element of risk into an already combustible mix.-BBC