Iran’s supreme leader, Ayatollah Ali Khamenei, said Tuesday he was content with ongoing talks with the United States but cautioned that the negotiations might not yield any results.
Tehran and Washington are due to meet again in Muscat on Saturday, a week after top officials held the highest-level talks since the landmark 2015 nuclear accord collapsed.
U.S. President Donald Trump, who pulled out of the deal during his first term, revived his "maximum pressure" sanctions campaign after returning to office in January.
In March, he sent a letter to Khamenei urging talks and warning of possible military action if Iran refused.
Saturday's talks were "well carried out in the first steps," Khamenei said, quoted by state television. "Of course, we are very pessimistic about the other side, but we are optimistic about our own capabilities."
But he added that "the negotiations may or may not yield results."
Despite having no diplomatic ties since Iran's 1979 Islamic Revolution, both sides described the talks as "constructive."
Iran insists discussions remain "indirect" and mediated by Oman.
On Monday, Trump again threatened to strike Iran's nuclear facilities if no deal was reached, calling Iranian authorities "radicals" who should not possess nuclear weapons.
Tehran denies seeking an atomic bomb, saying its nuclear programme is for peaceful purposes, especially energy production.
Khamenei said Iran's "red lines are clear," without elaborating.
Earlier Tuesday, Iran's Islamic Revolutionary Guard Corps said the country's military capabilities were off limits in the discussions.
"National security and defence and military power are among the red lines of the Islamic Republic of Iran, which cannot be discussed or negotiated under any circumstances," Guards spokesperson Ali Mohammad Naini said, quoted by state broadcaster IRIB.
Late Sunday, Iran's official IRNA news agency said the country's regional influence and its missile capabilities – long criticized by Western governments – were among its "red lines" in the talks.
Tehran supports the "axis of resistance" – a network of armed groups opposed to Israel, including Yemen's Houthi rebels, the Hezbollah armed group in Lebanon, the Palestinian resistance group Hamas in the Gaza Strip and Shiite militia groups in Iraq.
Iran has long been wary of talks with the United States, citing past mistrust.
The 2015 accord – known formally as the Joint Comprehensive Plan of Action, or JCPOA – offered Iran relief from international sanctions in return for curbs on its nuclear program.
Iran complied with the agreement for a year after Trump's withdrawal before scaling back its compliance.
In his speech, Khamenei said Iran should not pin its hopes on progress in the negotiations.
"At the time (of the JCPOA), we made everything conditional on the progress of the negotiations," he said.
"This mistake ... should not be repeated here."
