Iran's foreign minister kept his cards close to his chest Tuesday as he returned to the fray in marathon nuclear talks with major powers after consultations in Tehran, saying only he wants a "just" deal.
A deadline of midnight (2200 GMT) for an accord ending a dangerous 13-year standoff over Iran's nuclear ambitions was widely expected to be extended, but only for a few more days.
In what may be a positive sign, Russian Foreign Minister Sergei Lavrov was also due to arrive, as was German counterpart Frank-Walter Steinmeier, sources said, joining US Secretary of State John Kerry.
French Foreign Minister Laurent Fabius said Monday that he would return to Vienna this week. It was unclear when counterparts from Britain and China might arrive.
"The only agreement that the Iranian nation will accept is a just and balanced one (respecting) the national greatness and the rights of the Iranian people," Mohammad Javad Zarif told Iranian media before going into a one-on-one meeting with Kerry.
"All the officials have said they are ready for... a fair and rational trade-off," Zarif said, saying an agreement was "possible".
Zarif's return to Tehran on Sunday night after a weekend of intense negotiations with Kerry and other foreign ministers had sparked speculation that he would return with clear guidance that would enable an accord to be sealed.
It was unclear if he had. Zarif was however accompanied on his plane by Iran's nuclear chief Ali Akbar Salehi, as well as President Hassan Rouhani's brother and close confidante Hossein Fereydoun.
Diplomats say Salehi and his US counterpart Energy Secretary Ernest Moniz played a key role in drawing up a framework deal in Lausanne, Switzerland in April that was a major breakthrough.
Under that framework, Iran agreed to substantially scale down its nuclear activities in order to make any attempt to develop nuclear weapons -- an aim denied by Tehran -- virtually impossible.
In return painful sanctions that have suffocated the Iranian economy by choking its lifeblood oil exports and its ability to earn foreign currency will be progressively lifted.
But turning the 505-word joint statement drawn up in a Swiss lakeside hotel into a fully-fledged, highly technical document of several dozen pages and around five annexes, has proved hard work.
"It sounds easy, but it's difficult," Steinmeier had said Sunday.
A US official warned Monday "there are real and tough issues that remain which have to be resolved" as Kerry said "it's too early to make any judgements."
Key sticking points include the pace and timing of sanctions relief, the mechanism for their "snapback" and Iran's future development of newer, faster centrifuges.
Another thorny topic is the role of the International Atomic Energy Agency (IAEA) watchdog, whose chief Yukiya Amano met Kerry on Monday and who has been closely involved in the talks this week.
Under the mooted deal, it will be up to the IAEA, which already keeps close tabs on Iran's declared nuclear sites with between four and 10 inspectors on the ground on any given day, to verify that Iran really does reduce its capacities.
But the P5+1 also want the IAEA to have wider inspection rights to verify any suspicious activity that might indicate work in secret on a nuclear bomb.
This could include the IAEA visiting military bases, something that Iran's supreme leader Ayatollah Ali Khamenei last week rejected as a "red line".
A probe into allegations of such activity, before 2003 and possibly since, is stalled but clearing up these claims is a key condition of the six powers for a deal.
"It is important to get all the details right so that there are no ambiguities or weaknesses that can be exploited by critics of an agreement on both sides," said Arms Control Association expert Kelsey Davenport.
"This is an historic moment -- both sides have come too far to walk away without a good deal," she told AFP.