Rollercoaster talks with Iran stretched deep into the early hours of Thursday as the country's foreign minister told world powers to "seize the moment" to clinch a ground-breaking deal with Tehran to curtail its nuclear programme.
"We are a few metres... from the finishing line, but we are well aware that the final metres are the hardest," French Foreign Minister Laurent Fabius told reporters, as he arrived back in Switzerland to rejoin the negotiations.
The stakes were very high, he said, adding at issue was the question of non-proliferation, and "Iran's reintegration into the international community."
Fabius was re-joining US Secretary of State John Kerry and their counterparts from Germany and Britain, as well as political directors from Russia and China.
A US official confirmed Kerry held four hours of late-night talks with his Iranian counterpart Mohammad Javad Zarif and EU deputy foreign policy chief Helga Schmid, adding to days of already bruising negotiations.
In back-to-back talks, the top US diplomat then went straight into a meeting with Fabius and German Foreign Minister Frank-Walter Steinmeier at 1:20 am (2320 GMT), before just a few minutes later resuming negotiations with Zarif and Schmid.
"Iran has shown its readiness to engage with dignity and it's time for our negotiating partners to seize the moment," Zarif told reporters earlier.
Iran had shown it wants "an entente" with the world, but it "will not accept submitting to force and excessive demands," Zarif said.
After 18 months of intense negotiations, it remained unclear if the six world powers and Iran will pin down the main contours of a deal to put a nuclear bomb out of Iran's reach.
The aim is to turn this into a comprehensive accord backed by specific technical commitments by June 30 when an interim deal struck in November 2013 -- which saw Iran freeze certain nuclear activities in return for minor sanctions relief -- expires.
Success would end a 12-year-old standoff. Failure may set the United States and Israel on a road to military action to thwart Iran's nuclear drive.
The White House said the talks were still "productive" and progress was being made.
"But if we are in a situation where we sense that the talks have stalled then yes, the United States and the international community is prepared to walk away," said White House spokesman Josh Earnest.
- 'We will come to a conclusion' -
Despite missing Tuesday's midnight deadline, Russian Foreign Minister Sergei Lavrov had raised hopes of "an agreement in principle on all key aspects of the final settlement" before leaving on Wednesday.
Iran's chief negotiator Abbas Araghchi had also appeared hopeful, saying he believed "that at the end of the day we will be able to come to a conclusion and a resolution for all issues."
He said there were two main sticking points -- a mechanism for lifting crippling sanctions against the Islamic republic, and the country's research and development of new nuclear machinery.
Global powers want Iran to scale down its nuclear programme to extend the "breakout" time needed to assemble enough nuclear material to make a bomb.
Iran denies wanting the bomb and its negotiators are under strict orders from supreme leader Ayatollah Ali Khamenei to refuse any curtailing of its programme without sanctions relief.
Global powers have always refused an immediate lifting of all sanctions, preferring instead a phased suspension to enable them to be put back in place if Iran violates the deal.
If the outlines under negotiation fall short of firm commitments by Iran, US President Barack Obama could find it hard to fend off attempts by his Republican opponents to pass fresh sanctions on Tehran.
Iran's negotiators are also under pressure from domestic hardliners not to give too much away -- while also delivering on President Hassan Rouhani's promise to win the lifting of sanctions.
US Republicans fear that Iran will still be able to get the bomb -- a concern shared by Israeli Prime Minister Benjamin Netanyahu, whose country is widely assumed to have nuclear weapons itself.
Netanyahu, in his fourth broadside in as many days, warned against a bad deal that would "endanger Israel, and the Middle East and the peace of the world."