An Iranian man reads a copy of the daily newspaper 'Omid Javan' bearing a picture of US President Donald Trump with a headline that reads in Persian 'Crazy Trump and logical JCPOA (Joint Comprehensive Plan of Action)', on October 14, 2017, in front of a kiosk in the capital Tehran.

Iran will inform the U.N. nuclear watchdog in Vienna on Tuesday over its start of a process to increase the country's uranium enrichment capacity, Iran's Atomic Energy Organisation spokesman Behrouz Kamalvandi told ISNA news agency.

"In a letter that will be handed over to the International Atomic Energy Agency (IAEA) ... Iran will announce that the process of increasing the capacity to produce ... UF6 (uranium hexafluoride) ... will start on Tuesday," Kamalvandi said.

He said Iran had the capacity to accelerate production of centrifuges, which are used to enrich uranium. The UF6 is a feedstock for centrifuges.

Iran's ultimate authority Supreme Leader Ayatollah Ali Khamenei said on Monday he had ordered preparations to increase uranium enrichment capacity if a 2015 nuclear deal with world powers falls apart after the U.S. withdrawal from it last month.