Parties trying to revive the Iran nuclear deal on Wednesday scrambled to resolve last-minute Russian demands that threaten to scupper negotiations, diplomats said.
The US appeared unwilling to engage with Russia on the matter, Reuters reported.
Western powers on Tuesday warned Russia against wrecking an almost completed deal on bringing the US and Iran back into compliance with the 2015 accord. Iran's top negotiator returned to Vienna on Wednesday from consultations in Tehran.
Iran's top security official, Ali Shamkhani, said on Thursday the US does not have the will to reach an agreement.
Mr Shamkhani said on Twitter that in the absence of a political decision by the US the talks "become more complicated every hour".
The US “has no will to reach a strong agreement", he said. It was making “unacceptable proposals [and] insists on a quick agreement with false pretences”, he said.
He did not elaborate on the US proposals.
Russia's envoy to the talks, Mikhail Ulyanov, dismissed any suggestion Moscow was holding up an agreement and said a final text had in any case not been completed.
Eleven months of talks to restore the deal, which lifted sanctions on Iran in return for curbs on its nuclear programme, have reached their final stages with several diplomats saying there was broad agreement.
But just as the final issues were being resolved, Russia presented a new obstacle by demanding written guarantees from the US that western sanctions against Moscow over its invasion of Ukraine would not affect its trade with Iran.
Mr Ulyanov said Moscow's demands had not received a positive reaction.
"In view of the new circumstances and wave of sanctions against Russia we have the right to protect our interests in the nuclear field and wider context," Mr Ulyanov said.
He said the US and the EU had to make it clear that sanctions, neither now nor in the future, could hit the implementation of nuclear projects in Iran, as well as its trade and economic relations.
Mr Ulyanov met the co-ordinator of the talks, Enrique Mora of the EU, on Tuesday evening and again on Wednesday.
He said he would still have to report back to Moscow for a final decision after the text was finalised.
"There is no final text, so how can our position delay anything if final negotiations are not finished," he said. "A number of participants at this moment are not ready to confirm that the text is fully acceptable to them."
US Under Secretary of State for Political Affairs Victoria Nuland on Tuesday accused Russia of seeking to reap extra benefits from its participation in the effort to restore the nuclear agreement, but she said Washington would not be playing "Let's Make a Deal".
Two western diplomats said it was still not clear what the exact nature of Moscow's demands were, while a European diplomat said Russia was demanding sweeping guarantees on trade between Moscow and Tehran, demands that were deemed unacceptable.
They said the talks were now not likely to end this week.
