"Iran is willing but will not wait forever," Khatibzadeh wrote in a tweet on Tuesday.
Efforts to revive the 2015 nuclear deal could succeed if the United States took a political decision to meet Tehran's remaining demands, he said.
The stakes are high, since the failure of 10 months of talks could carry the risk of a fresh regional war, of more harsh sanctions on Iran by the West and of continued upward pressure on world oil prices already strained by the Ukraine conflict.
All parties involved in the talks say progress has been made toward the restoration of the pact - which the United States abandoned in 2018 - to curb Tehran's nuclear program in exchange for sanctions relief.
But there remain some significant differences to overcome.
"Reaching a good deal is possible," Khatibzadeh told a weekly news conference while saying Western powers had failed to take "political decisions" on the outstanding issues.
France's foreign ministry said on Monday it was urgent to conclude the talks this week.
Iran's lead nuclear negotiator Ali Bagheri Kani, who flew to Tehran last week for consultations about the final draft of the deal, met the European Union's Enrique Mora, who co-ordinates the talks in Vienna, on Monday.
Two sources close to the talks said Iran had submitted new demands, while continuing to insist on existing ones, including the removal of a US foreign terrorist organisation (FTO) designation against Iran's elite Revolutionary Guards (IRGC).
"Iran's stance after Bagheri's trip to Tehran has become even more uncompromising," said one of the sources.
"They now insist on removal of sanctions on the IRGC and want to open issues that had already been agreed."
Iran has previously said removing the guards from the terrorism list was under discussion. Iranian officials did not immediately respond to a request for comment.
The IRGC is a powerful faction in Iran, controlling a business empire as well as elite armed forces and intelligence services. Its 2019 FTO designation by Washington has caused further problems for Iran's sanction-hit economy.
Tehran also insists the International Atomic Energy Agency (IAEA) drop its claims about Tehran's nuclear work, objecting to last year's assertion by the nuclear watchdog that Tehran had failed to fully explain uranium traces at undeclared sites.
Then-US president Donald Trump pulled the United States out of the pact in 2018 and reimposed tough economic sanctions.
That led Iran to breach the nuclear limits of the deal, which was designed to make it harder for Tehran to obtain the fissile material for a nuclear bomb. Iran denies any such ambition.
Diplomats said the negotiations have entered a crucial stage, given Iran's uncompromising policy in the talks and the other parties' involvement in the crisis over Ukraine.
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