Tehran and Washington held their fourth round of nuclear talks on Sunday -- their highest-level contact since the United States pulled out of the 2015 nuclear deal in 2018 during Donald Trump's first term as US president.

"For a limited period of time, we can accept a series of restrictions on the level and volume of enrichment," said Deputy Foreign Minister Majid Takht-Ravanchi.

"We have not yet gone into details about the level and volume of enrichment," he said, quoted by Tasnim news agency.

Iran currently enriches uranium to 60 percent purity -- far above the 3.67 percent limit set in the 2015 deal but below the 90 percent needed for weapons-grade material.

US Secretary of State Marco Rubio said Monday that Iran was the only country in the world without nuclear weapons that enriches uranium to that level.

Iranian Foreign Minister Abbas Araghchi said at the latest talks that the right to enrich uranium was "non-negotiable", while US chief negotiator Steve Witkoff called it a "red line".

The Islamic republic began rolling back its commitments to the deal a year after the US withdrawal.

Since returning to office in January, Trump has revived his "maximum pressure" approach against Tehran. While backing nuclear diplomacy, he also warned of potential military action if it fails.

Western countries, including the United States, have long accused Iran of seeking to acquire nuclear weapons, while Iran insists its nuclear programme is for peaceful purposes.

Iran described Sunday's talks as "difficult but useful" while a senior US official said Washington was "encouraged" and both sides confirmed plans for future negotiations.

On Tuesday, Iran's atomic energy agency chief, Mohammad Eslami, described the country's nuclear industry as its "wealth and strength", according to ISNA news agency.

Despite the talks, Washington has continued to impose sanctions targeting Iran's nuclear programme and oil industry, with the latest announced on Monday.

"There is no doubt that there is a lot of pressure on us," said Ali Larijani, a close adviser to Iran's supreme leader, while noting that not all of Iran's problems were due to the sanctions.