"Being near the finish line is no guarantee to crossing it" and sealing an accord, Iran's chief negotiator Ali Bagheri Kani says.
Western partners in the nuclear talks with Iran in Vienna have to "take certain decisions" in order to reach an agreement, Iran has said.
"Being near the finish line is no guarantee to crossing it" and sealing an accord, Iran's chief negotiator Ali Bagheri Kani said on Thursday.
"It requires extra caution, much perseverance, additional creativity and balanced approach to take the last step," he added.
Iran on Wednesday urged Western powers to be "realistic" in the talks and said Bagheri Kani was returning to Tehran for consultations, suggesting a breakthrough in its discussions is not imminent.
Talks in crucial stage
Diplomats from parties involved in the negotiations have said they have entered a crucial stage, with a Russian envoy saying that the discussions are nearing a conclusion.
The deal on Iran's nuclear programme is expected before the end of the week with some minor technical issues being discussed in Vienna, several Iranian officials have said on Tuesday.
Diplomats said a vague mention of other issues in a draft text of the agreement was a reference to the unfreezing billions of dollars in Iranian funds in South Korean banks, and the release of Western prisoners held in Iran.
Sources close to the negotiations have said a prisoner swap between Iran and the United States was expected soon.
The US-Iranian deal has been taking shape to revive the pact, abandoned in 2018 by then-US President Donald Trump, who also reimposed extensive sanctions on Iran.
The 2015 deal between Iran and world powers limited Tehran's enrichment of uranium to make it harder for it to develop material for nuclear weapons, if it chose to, in return for a lifting of international sanctions against Tehran.