The longtime foes have held five rounds of talks since April, the highest level contact since President Donald Trump withdrew Washington from a 2015 nuclear accord during his first term.

"The next round of Iran-US indirect talks is being planned for next Sunday in Muscat," Iran's foreign ministry spokesman Esmaeil Baqaei said in a statement Tuesday.

Trump had earlier said the meeting with Iran was expected on Thursday but Baqaei noted that Iran's foreign minister and chief negotiator Abbas Araghchi would be attending the Norway's Oslo Forum, a gathering of conflict mediators.

The sixth round of talks will come around two weeks after Iran received a US proposal for nuclear deal which Araghchi described as containing "ambiguities".

Iran said the US proposal failed to include "elements" reflective of the previous negotiations including lifting of sanctions -- a key demand for Tehran, which has been reeling under their weight for years.

Tehran said it would present a "reasonable, logical and balanced" counter-proposal to the United States through mediator Oman.

On Monday, Trump said the upcoming US-Iran talks could clarify if a nuclear deal is possible to avoid military action.

Iran and the United States have been locked in a diplomatic standoff over Iran's uranium enrichment, with Tehran defending it as a "non-negotiable" right and Washington calling it as a "red line".

- 'National interests' -

Iran currently enriches uranium to 60 percent, far above the 3.67-percent limit set in the 2015 deal and close though still short of the 90 percent needed for a nuclear warhead.

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

On Tuesday, Iran's deputy foreign minister Majid Takht-Ravanchi dismissed claims that there was a "deadlock" in the talks with the US.

"Any negotiations with an international dimension come with their own sensitivities and require a great deal of patience for them to reach a result," he told the official IRNA news agency.

"What matters to us is that we can safeguard the national interests of the country through these talks, and that is how we proceed," he added.

He said that the sixth round will be the first in which the parties express their views on the exchanged written materials, noting that Tehran will again reiterate its position on enrichment capacity, as it has done publicly in the past.

On Monday, the United Nations nuclear watchdog began a Board of Governors meeting in Vienna that will last until Friday to discuss Iran's nuclear activities among other topics.

The meeting came after the International Atomic Energy Agency released a report criticising "less than satisfactory" cooperation from Tehran, particularly in explaining past cases of nuclear material found at undeclared sites.

Iran has criticised the IAEA report as unbalanced, saying it relied on "forged documents" provided by its arch foe Israel.

On Tuesday, Araghchi reiterated criticism of a plan by European powers and the United States to adopt a resolution at the IAEA meeting, accusing Tehran of non-compliance with its nuclear obligations.

"Any ill-considered and destructive decision in the Board of Governors against Iran will be met with an appropriate response," he said during a phone call with Japanese Foreign Minister Takeshi Iwaya.

Iran has said it would reduce cooperation with the IAEA if the resolution passed.