Tehran, Iran - Iran remains optimistic that talks about its nuclear program will yield a result that works for all sides, despite a rocky return to negotiations last week and criticism from the other parties, according to a Foreign Ministry spokesperson.

"That was just the first meeting and no one could have seriously expected a concrete result," Saeed Khatibzadeh said at a press conference in Tehran on Monday, adding that Iran wanted to reach an agreement and would remain "determined, but flexible" when talks resume later this week.

The talks in Vienna are designed to resurrect the 2015 deal, in which Iran agreed not to pursue nuclear weapons in exchange for an end to international sanctions.

However, former President Donald Trump pulled the US out of the deal unilaterally and resumed sanctions, after which Iran restarted its nuclear program and stopped complying with some aspects of international inspection arrangement.

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Last week's return to negotiations did not augur well, with Iran refusing direct talks with the US and both sides criticizing one another. The White House accused Iran of not being seriously interested in a new deal. Other partners to the talks also expressed disappointment.

"We put forward two proposals and are now waiting for answers," said Khatibzadeh.

He said both suggestions are in line with the 2015 deal and pertain to a lifting of US sanctions and Iran's return to the deal's technical requirements.

"Those are the two key points without which the continuation of the talks aren't possible," he said. Instead of criticizing Iran, the US and its European partners should simply return to the original deal and implement it, Khatibzadeh added.