Iran will continue to enrich uranium to 20% purity even after sanctions on it are lifted and the Joint Comprehensive Plan of Action (JCPOA) nuclear deal from 2015 is reinstated after the United States had unilaterally pulled out of it and reimposed sanctions on Iran, the country's nuclear chief said on Friday.

"(Uranium) enrichment ... continues with a maximum ceiling of 60%, which led Westerners to rush to negotiations, and it will continue with the lifting of sanctions by both 20% and 5%," the head of Iran's Atomic Energy Organization, Mohammad Eslami, was quoted by the semi-official news agency Fars as saying.

The 2015 deal restricts the purity to which Iran can enrich uranium to 3.67%, far below the roughly 90% that is weapons-grade or the 20% Iran reached before the deal. Iran is now enriching to various levels, the highest being around 60%.

After 10 months of talks in Vienna, progress has been made toward the restoration of the pact to curb Tehran’s nuclear program in exchange for sanctions relief, but both Tehran and Washington have cautioned that still there are some significant differences to overcome.

The 2015 deal between Iran and world powers limited Tehran's enrichment of uranium to make it harder for it to develop material for nuclear weapons, if it chose to, in return for a lifting of international sanctions against Tehran.

The main remaining disputes appear to include the extent of sanctions rollbacks and questions about uranium traces found at several old but undeclared sites in Iran.

Iran has made clear it wants an end to the oil and banking sanctions crippling the economy, while insisting also on the lifting of human rights- and terrorism-related curbs.