Officials from both sides acknowledged on Sunday that they would probably not be able to seal a final comprehensive deal to curb Iran's nuclear program in return for an easing of economic sanctions by their self-imposed deadline of midnight on Tuesday.

Late on Sunday, Iranian Foreign Minister Mohammad Javad Zarif left Vienna on a flight home to Tehran for consultations with his government, but was expected to return to the Austrian capital for further talks in the next few days.

Despite the looming deadline, the various delegations seemed relaxed that getting a deal was going to take longer than hoped, expressing confidence that they would get an agreement.

The European Union's foreign policy coordinator, Federica Mogherini, said the two sides had produced "good results" in a day of tough negotiations.

"I would say that the political will is there. I've seen it from all sides," Mogherini said. "So that we've tasked negotiating teams to continue work immediately tonight on the texts [for an agreement]."

"There's a good deal, and we have to use these hours these days to do it. Postponement is not an option," she added, noting that even though Zarif and some of the other foreign ministers were leaving Vienna for now, they had left their delegations with political guidelines on how to move forward.

Referring to the issues that still needed to be ironed out, Mogherini said they related mainly to how to "translate the political understandings that we reached already into concrete detail."

At loggerheads over inspections

The main sticking point appeared to be Iran's willingness - or lack thereof - to grant inspectors from the United Nations' atomic watchdog agency to its nuclear facilities.

German Foreign Minister Frank-Walter Steinmeier, though, indicated that the P5+1 group, which also includes the United States, Britain, France, Russia, and China, was not prepared to be flexible on inspections.

"The issue of if and where monitoring is done cannot be called into question," Steinmeier said, adding that the P5+1 would only show flexibility on how such visits would be conducted.

"There are red lines that we cannot cross, and some very difficult decisions and choices are going to have to be made by all of us," British Foreign Secretary Philip Hammond said.

The two sides are hoping to seal a comprehensive deal along the lines of a framework agreement reached in the Swiss city of Lausanne on April 2, which would see Iran accept curbs to its nuclear program designed to ensure that it cannot acquire a nuclear weapon. In exchange, the international community would gradually relax sanctions that have crippled Iran's economy for years.

pfd/bk (dpa, AFP, AP, Reuters)