It comes as another round of talks ended inconclusively and the US State Department has reminded negotiating parties the new deadline for Iranian compliance was looming and would not be changed.
State Department spokesman Sean McCormack said Iran had not agreed to suspend its uranium enrichment program in the discussions.
Talks are being conducted between Iran’s top nuclear negotiator, Ali Larijani, and European Union foreign policy chief Javier Solana in Berlin.
The new deadline was agreed to last week among the permanent five UN Security Council members and Germany, dubbed P-5-plus-1 nations.
"The timeline that was agreed in New York stays, and we are getting short now in terms of that time," Mr McCormack said, referring to the moment when the six nations would seek UN sanctions against Tehran.
Sanctions threatened
The deadline has not been officially revealed by the six nations, which also include Britain, China, France, Russia and the United States, but European diplomats involved in the negotiations said it was sometime next week.
"The ball is in their court," Mr McCormack said Thursday of the Iranians.
"Nobody wants to go down the path of sanctions -- that is not our first choice," he said.
"But we are prepared, along with the P-5-plus-1, to go down that path if that's the door that the Iranian regime wants to open," he said.
Iran has already ignored an August 31 deadline set by the UN Security Council for it to suspend uranium enrichment and reprocessing activities.
Washington and others believe those activities are a cover for developing nuclear weapons.
Tehran insists the program is only for producing fuel for nuclear power stations.
The UN resolution allows for sanctions, but these would have to be elaborated in another set of UN documents.
Mr McCormack said that Washington is currently working on those documents with its Security Council partners in parallel with the Solana-Larijani talks.
‘Positive’ discussions
Mr Solana and Mr Larijani have characterised their latest two days of discussions as positive and constructive, but said the two sides would not be in touch again next week.
US Secretary of State Condoleezza Rice spoke with Solana about the talks on Wednesday night and was expected to get another briefing from the EU official Thursday, Mr McCormack said.
The EU-Iran talks focus on a list of economic and diplomatic incentives put forward by the six powers in June as a reward if Iran suspends its uranium enrichment.
The incentives include support for an Iranian nuclear power industry and the first direct contacts between the US and Iran in nearly 30 years.
Mr McCormack insisted that negotiations involving the United States could begin only after Iran freezes its enrichment program and the suspension is verified by monitors from the UN nuclear watchdog, the International Atomic Energy Agency.