America's top negotiator leading talks to curb Tehran's nuclear ambitions Thursday called on Iranian leaders to "make the right choice," saying now was the time to reach a deal.

"If Iran truly wants to resolve its differences with the international community -- and facilitate the lifting of economic sanctions -- it will have no better chance than between now and November 24," Undersecretary of State Wendy Sherman said.

"This is the time to finish the job."

The six powers leading the talks with Iran, known as the P5+1, have set the November deadline to reach a comprehensive deal.

The aim is to close all of Tehran's avenues towards developing an atomic bomb, by cutting back its nuclear enrichment program, shutting down suspect facilities and imposing tough international inspections.

In return, the global community would suspend and then gradually lift crippling economic sanctions imposed on the Islamic republic.

"Will that happen?" Sherman asked, addressing an audience at a two-day symposium organized at the Center for Strategic and International Studies think-tank.

"I don't know. I can tell you that all the components of a plan that should be acceptable to both sides are on the table."

As usual, she gave no details about the complex draft accord being hammered out by teams of experts, complete with complicated pages of technical annexes.

"Our bottom line is unambiguous, crystal-clear, and written-in-stone: Iran will not, shall not obtain a nuclear weapon," Sherman pledged.

But the P5+1 has put forward a number of ideas recognizing "Tehran's expressed desire for a viable civilian nuclear program and that take into account that country's scientific knowhow and economic needs."

In a bid to break the stalemate, US Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif held six hours of talks in a Vienna hotel last week.

"We have made impressive progress on issues that originally seemed intractable," Sherman insisted.

But she stressed the negotiations were "a puzzle with many interlocking pieces" and were "the very opposite of easy."

The talks were still at a "difficult stage."

"We must use the remaining time wisely and with a sense of urgency and purpose," Sherman said.

So far the Americans have said they are not even talking about extending the deadline again after an early July date for a final deal was missed.

Many observers believe however that a further extension will be unavoidable, as all sides have invested too much to simply walk away if they can't agree every detail in time.

"We encourage Iran to make the right choice. Meanwhile, we remain steadfast in our determination to take the steps necessary to protect America's security and to improve the prospects for stability and peace across the globe," Sherman added.