But a senior Iranian official warned that negotiations are hampered by differences not only between Tehran and the six other countries it is bargaining with but internally among the six as well.
Kerry was the first to arrive Friday evening, with diplomats from Iran, Britain, Germany, Russia, France and China expected in the Austrian capital over the coming days.
Negotiations could spill over into early July, despite the extra diplomatic muscle packed by the senior diplomats. Ahead of Kerry's arrival, Iranian Deputy Foreign Minister Abbas Araghchi, Iran's top negotiator, told Iran's IRNA news agency that — while "on the whole we are making headway" — progress was slow and hard.
Iran and the six countries at the negotiating table agreed in April on the outlines of a deal that would crimp Tehran's nuclear activities for at least 10 years in return for sanctions relief for Tehran. But negotiators are finding it difficult to fine tune which sanctions should be lifted when and how open Iran must be to outside monitoring.
Complicating issues, said Araghchi, was the fact that the nations Iran is negotiating with also differ in their approach to certain topics, "which may not be harmonized easily." He did not offer details.
Signs that Iran is toughening its stance add to the likelihood that the talks could go past the target date. In a speech this week, Iran's supreme leader, Ayatollah Ali Khamenei, rejected a long-term freeze on nuclear research and insisted that Iran will only sign a deal if international sanctions are lifted first.
He also insisted that military sites and Iranian nuclear scientists will be off-limits to U.N. experts, who would monitor Iranian compliance to any deal while trying to kick-start a moribund probe of suspicions that Tehran worked on atomic arms — allegations that Iran denies.
Kerry subsequently suggested Khamenei was speaking to a domestic audience appreciative of a hard-line stance and said that a deal would be out of reach if Tehran also reneges on its commitments at the negotiating table. Still, Khamenei's comments added to skepticism among some U.S. legislators who already believe the Obama administration has grossly backslid on its positions in efforts to achieve an agreement.
Democratic Senator Bob Menendez, a key critic of the negotiations, called Khamenei's demands "unacceptable," in a letter to Kerry on Friday.
"If Iranian negotiators intend to adhere to the provisions demanded by Ayatollah Khamenei and Iran's parliament, I urge you to suspend the current negotiations with Iran," he said.
Araghchi said the negotiators were still aiming for a June 30 deal. But the more important target date is July 9. As part of an agreement with U.S. Congress, if the legislature receives a deal by then, it has 30 days to review it before President Barack Obama could suspend congressional sanctions.
But postponement beyond that would double the congressional review period to 60 days, giving both Iranian and U.S. opponents more time to work on undermining an agreement.