The hoped-for agreement would see Iran scale back its nuclear programme, in order to ease fears Iran wants atomic weapons, and avert a conflict in the Middle East.
Iran, which has seen its relations with the West thaw somewhat since the 2013 election of President Hassan Rouhani, wants painful UN and Western sanctions lifted. It denies wanting the bomb.
On a fourth day of talks in Vienna, Iran and the five permanent members of the UN Security Council plus Germany have started haggling over the wording of a deal, officials said.
Yesterday negotiations "slowly" began to draft the final agreement, "but there are still many differences" over the text, ISNA news agency quoted Zarif as saying from Vienna.
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He added that the talks had been "very difficult".
A Western diplomat said that Iran was refusing to budge on most issues and that drafting language in the text on the "complex issues" had not begun.
"It is worrying that there is no evolution on the part of the Iranians on most subjects," the diplomat told AFP on condition of anonymity.
Enrichment is front and centre of Western concerns about Iran's nuclear ambitions, as the process can produce both fuel for nuclear power plants and, when highly purified, the core of an atomic bomb.
The West wants Iran to slash the number of centrifuges, the machines that enrich uranium, from the current 20,000, but Tehran wants to install many more in order, it says, to fuel a future fleet of nuclear plants.
"Bearing in mind the limited time that is left and the differences remaining, the progress is slow" in writing the draft, a senior Iranian diplomat at the talks told ISNA.