"We have of course adopted some measures in order to prepare the ground for eventually increasing the level of enrichment if it is needed and if the negotiations with the Europeans fail," Behrouz Kamalvandi, spokesman and vice-president of Iran's Atomic Energy Organisation, told a news conference in Tehran.

"We are of course continuing to carry out and implement our obligations based on the JCPOA," he said, referring to the 2015 nuclear deal with world powers that put strict limits on its atomic programme in return for sanctions relief.

"But at the same time, taking every scenario into consideration, we are preparing ourselves," he added.

The United States announced in May that it was abandoning the 2015 agreement and reimposing nuclear-related sanctions, threatening global companies with heavy penalties if they continue to operate in Iran.

In a bid to save the accord, the EU and European parties to the deal -- Britain, France and Germany -- presented a series of economic "guarantees" to Iran this month, but these were judged "insufficient" by Tehran.

Negotiations are continuing, and foreign ministry spokesman Bahram Ghasemi said Monday they could last several "weeks", according to state television.

In June, in a bid to mount pressure on the Europeans, Iran announced a plan to increase its uranium enrichment capacity with new centrifuges in the event that the agreement collapses, while still denying any desire to build a nuclear weapon.

Under the 2015 agreement, Iran can only enrich uranium to 3.67 percent -- far below the roughly 90-percent level needed for nuclear weapons.