Iran has multiplied the speed at which it enriches uranium but it is still far from the maximum rate possible under its nuclear deal with major powers, meaning it would be months before production ceilings are hit, diplomats who follow it say.

Against a backdrop of rising tensions with the United States, which pulled out of the 2015 deal a year ago and has since imposed ever more punishing economic sanctions against Tehran, Iran has stayed within the deal's key limits while threatening to discard at least some of them.

Since it withdrew from the deal, Washington has reinstated its sanctions against Tehran and added new ones in a bid to isolate the Islamic Republic — an effort Iran calls "economic terrorism."