Time is running short to reach a nuclear deal between world powers and Iran, and negotiations will not be extended merely as a foot-dragging ploy, top US diplomat John Kerry warned Tuesday.

On the eve of the most intensive round of talks yet, Kerry called on Iran to make the right choices and prove to the world its claims that its nuclear energy program is peaceful by closing what he called "substantial gaps" in the negotiations.

"We have worked closely with Iran to design a pathway for a program that meets all of the requirements for peaceful, civilian purposes," Kerry wrote in an op-ed in The Washington Post.

"There remains a discrepancy, however, between Iran's professed intent with respect to its nuclear program and the actual content of that program to date."

Western nations have long accused Tehran of seeking to develop an atomic bomb -- something the leaders of the Islamic republic have vehemently denied.

Talks between Iran and the so-called P5+1 group to strike a deal disabling any Iranian nuclear military program will resume on Wednesday in Vienna and are set to last until July 20 -- the deadline for reaching a full treaty.

Under an interim six-month deal struck in Geneva in November, cash-strapped Iran agreed to halt uranium enrichment and eliminate its stockpiles in return for limited sanctions relief.

The pact also contains a provision for a one-time six-month extension of the talks, if all sides agree. The P5+1 group includes the five permanent members of the UN -- Britain, China, France, Russia, and the United States -- plus Germany.

"Our negotiators will be working constantly in Vienna between now and July 20," Kerry vowed.

"There may be pressure to put more time on the clock. But no extension is possible unless all sides agree, and the United States and our partners will not consent to an extension merely to drag out negotiations," he warned.

- Substantial gaps -

"Now Iran must choose," he said, adding that "time is running short."

"What will Iran choose? Despite many months of discussion, we don't know yet. We do know that substantial gaps still exist between what Iran's negotiators say they are willing to do and what they must do to achieve a comprehensive agreement."

The last round of talks in June were reportedly strained, and Iranian Foreign Minister Mohammad Javad Zarif has lashed out at world powers for making "excessive demands" on his country.

But Kerry countered that they had "proposed a series of reasonable, verifiable and easily achievable measures that would ensure Iran cannot obtain a nuclear weapon," and in return the Islamic republic "would be granted phased relief from nuclear-related sanctions."

"The world is simply asking Iran to demonstrate that its nuclear activities are what it claims them to be," Kerry added.

He acknowledged that Iranian negotiators had been "serious" so far throughout the talks, and that Iran had met obligations by working to eliminate its stockpiles of highly-enriched uranium, not installing additional centrifuges and allowing international inspections.

He argued there were not often chances to "make the world safer, ease regional tensions and enable greater prosperity."

"We have such an opportunity, and a historic breakthrough is possible. It's a matter of political will and proving intentions, not of capacity. It's a matter of choices. Let us all choose wisely."