US Secretary of State John Kerry said major differences persist between Iran and six world powers negotiating on Tehran's nuclear programme, with a week to go before a deadline for a deal.
The United States, Britain, France, Germany, Russia and China want Iran to reduce its nuclear fuel-making capacity to deny it any means of quickly producing atom bombs.
In exchange, international sanctions that have crippled the large OPEC member's oil-dependent economy would gradually be lifted.
Iran says it is enriching uranium for peaceful energy purposes only and wants the sanctions removed swiftly.
But a history of hiding sensitive nuclear work from UN inspectors raised international suspicions and the risk of a new Middle East war if diplomacy fails to yield a long-term settlement.
"Obviously we have some very significant gaps still, so we need to see if we can make some progress," Mr Kerry said ahead of meetings with foreign ministers who flew into the Austrian capital at the weekend to breathe new life into the talks.
Iranian Deputy Foreign Minister Abbas Araqchi delivered a similar message.
He was quoted by Iran's Arabic language al-Alam television as saying that "disputes over all major and important issues still remain. We have not been able to narrow the gaps on major issues and it is not clear whether we can do it."
Mr Kerry arrived in Vienna in the early hours after clinching a deal in Kabul with Afghanistan's presidential candidates to end the country's election crisis.
"It is vital to make certain that Iran is not going to develop a nuclear weapon and that their programme is peaceful and that's what we're here to try and achieve and I hope we can make some progress," Mr Kerry said in Vienna.
German Foreign Minister Frank-Walter Steinmeier told reporters that Germany and the other members of the six-power group have tried to persuade Iran of the urgency of a deal.
"This may be the last chance for a long time to peacefully resolve the dispute over Iran's nuclear programme," he told reporters.
"It's now up to Iran to decide whether it wants cooperation with the international community or to remain in isolation. ... The ball is in Iran's court."
British Foreign Secretary William Hague said it was crucial for Tehran "to be more realistic about what is necessary" to reach a nuclear deal, adding that no breakthroughs had been achieved and there was "no major change in the state of play in these negotiations as of this moment".