Top diplomats worked past a midnight deadline into the early hours of Wednesday seeking to agree the outlines of a nuclear deal with Iran, insisting "enough progress" had been made to continue the marathon talks.
"We've made enough progress in the last days to merit staying until Wednesday. There are several difficult issues still remaining," US State Department spokeswoman Marie Harf said.
The announcement came late on a sixth day of talks in Switzerland aimed at laying the groundwork for a deal that world powers hope will prevent Iran developing nuclear weapons under the guise of its civilian programme.
The stakes are high, with fears that failure to reach a deal may set the United States and Israel on a road to military action to thwart Iran's nuclear drive, which Tehran says is purely peaceful.
Senior Iranian negotiator Hamid Baidinejad also hinted at progress, saying "many of the issues linked to the sanctions have been resolved."
"We are in the final phase of the negotiations," he told Iranian television, adding there were still a few "limited" issues linked to sanctions to be resolved, along with questions about Iran's research and development.
Iran has been calling for an immediate lifting of the sanctions, while the six powers are only prepared to suspend sanctions to be able to put them back into place if Tehran violates the deal.
An army of technical and sanctions experts would continue plugging away "for (the) next hours. All parties (are) working hard and (are) committed to finding a solution," a Western diplomat said.
Earlier Chinese Foreign Minister Wang Yi left the crunch talks in Lausanne, although his team remained at the table, as his French counterpart Laurent Fabius said the negotiations were "complicated... long and difficult".
The return earlier to Lausanne of Russian Foreign Minister Sergei Lavrov had brought a ray of optimism, after he had left Moscow saying the chances of an accord were "high".
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Under a deal to be finalised by June 30, the powers want Iran to scale back its nuclear programme to give the world ample notice of any dash to make the bomb by extending the so-called "breakout" time.
In return, the Islamic republic is demanding the lifting of sanctions that have strangled its economy.
But the question is how much detail will be in the framework accord that Iran and the six powers -- the US, China, Russia, Britain, France and Germany -- want to nail down.
If it falls short of firm commitments by Iran then US President Barack Obama will 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 their own domestic hardliners not to give too much away and for President Hassan Rouhani to deliver on his promises to secure the lifting of sanctions.
Fresh US sanctions could therefore torpedo the whole negotiating process that was launched after Rouhani became president in 2013.
Republicans fear that since some of its nuclear infrastructure will likely stay intact, 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.
"The greatest threat to our security and our future was and remains Iran's attempt to be armed with nuclear weapons. The agreement being formulated in Lausanne paves the way to that goal," Netanyahu told parliament Tuesday.
"We will do everything to protect our security and our future," he said in his third attack on the nuclear talks with Iran in as many days.
Saudi Arabia, which has led an Arab coalition bombing Iran-backed rebels in Yemen in recent days, is also alarmed by what is unfolding in Lausanne.
Foreign Minister Prince Saud al-Faisal called on the six powers not to "short-circuit the interests of the states of the region by dangling before Iran benefits that it will not be able to reap without cooperation with the countries of the region."
Other areas of the mooted deal, including the future size of Iran's uranium enrichment capacity -- a process for making nuclear fuel but also the core of an atomic bomb -- also appear to have been tentatively sewn up.
But the two sides still appear to be discussing other areas, including what to do with Iran's stockpiles of nuclear material, and how long the deal should last.