Carlos Barria, Associated Press
A security personal asks reporters to leave a room where U.S. Secretary of State John Kerry, center, is meeting with Iranian Foreign Minister Mohammad Javad Zarif at a hotel in Vienna, Austria, Sunday, June 28, 2015.

VIENNA — Iran's chief diplomat insisted Tuesday he had a mandate to finalize a nuclear agreement despite increased signs of backtracking by his country's supreme leader, as talks with world powers were set to blow past Tuesday's self-imposed deadline without a deal.

Returning to the negotiations in Vienna, Iranian Foreign Minister Mohammad Javad Zarif said the diplomacy had reached a "very sensitive stage" but that progress was possible. Asked by a reporter about his meetings at home, he said: "I already had a mandate to negotiate and I am here to get a final deal and I think we can." He then continued his discussions with U.S. Secretary of State John Kerry.

Zarif returned with Ali Akbar Salehi, the head of Iran's atomic agency, who had missed earlier sessions due to illness. Iran's official news agency called Salehi's participation an indication of Iran's desire to accelerate talks. Russian Foreign Minister Sergey Lavrov was also expected to join the gathering Tuesday.

The negotiators hope to clinch an accord curbing Iran's nuclear program for a decade in exchange for tens of billions of dollars in relief from international economic sanctions. But significant disagreements persist, not least over the level of inspections on Iranian sites, how quickly the West will roll back sanctions, and what types of research and development Iran will be permitted to conduct on advanced nuclear technology.

On Monday, U.S. officials suggested that significant backtracking by Tehran's negotiators may need several more days to resolve. In recent weeks, as well, Ayatollah Ali Khamenei has issued a series of red lines that appear to renege on a framework for a deal his representatives agreed to three months ago in Lausanne, Switzerland.

Asked if he was encouraged by the restart of talks, Kerry said only, "We had a good conversation." The secretary of state, hobbled by a broken leg he suffered a month ago, has kept a low public profile since arriving in Austria last week.

Tuesday had originally been envisioned as the culmination of a 20-month process to assure the world Iran cannot produce nuclear weapons and provide the Iranian people a path of out of years of international isolation. But officials said over the weekend they were nowhere near a final accord, and Zarif flew back to his capital for further consultations.

The U.S., France and Iran have said there is no new target date for a deal, but that another in a series of long-term extensions wasn't being contemplated. American officials say the talks will likely stretch through the end of the week, possibly longer.