Iran's supreme leader Ali Khamenei said on Tuesday he was satisfied with talks with the United States but warned they could ultimately prove fruitless.
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.
US 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, according to AFP.
"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 program is for peaceful purposes, especially energy production.
Khamenei said Iran's "red lines are clear", without elaborating.
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."
