The world powers negotiating a nuclear deal with Iran have gone on the overdrive to clinch a framework agreement before the March 31 deadline, with the US, German and French representatives cancelling travel plans to attend the talks.
US Secretary of State John Kerry cancelled a planned return to the US to attend an event honouring his late Senate colleague Edward Kennedy, to ensure that he could attend Sunday's talks in the Swiss city of Lausanne, the US state department said, according to a BBC report.
German Foreign Minister Frank-Walter Steinmeier and his French counterpart Laurent Fabius also delayed a planned trip to Kazakhstan in order to focus on the negotiations.
They were joined by Chinese Foreign Minister Wang Yi and British Foreign Secretary Philip Hammond.
The P5+1 group of world powers -- US, Britain, France, Russia, China, plus Germany -- met Iranian Foreign Minister Mohammad Javad Zarif at Lausanne on Sunday.
US officials said all parties, including Iran, had agreed that "there needs to be a phased step-by-step reciprocal approach", so that Iran's steps to scale back its nuclear programme are met with a phased lifting of sanctions.
Two core issues remained on the table, they said. One was how the sanctions on Iran would be lifted; the other was what would happen in later years of the agreement, including Iran's capacity to conduct nuclear research and development.
"We've put ideas on the table, but we haven't found the right combination yet, but no one's given up," the officials said.
Zarif had said on Saturday that progress was being made.
"I think we can, in fact, make the necessary progress to be able to resolve all the issues and start writing them down in a text that will become the final agreement," he said.
British Foreign Secretary Philip Hammond said: "We're here because we believe a deal can be done, it's in everybody's interest that a deal does get done. But, it has to be a deal, which puts the bomb beyond Iran's reach. There can't be any compromise about that."
Potential sticking points in the nuclear talks are thought to include how long the deal will last and how much of Iran's nuclear facilities will be open to inspection.
The parties involved in the negotiations had extended the talks in November last year and are hoping for a comprehensive pact by the end of June this year.
Meanwhile, Israel's Prime Minister Benjamin Netanyahu has again warned of a deal with Iran, describing it as worse than his country had feared.
He accused Iran of trying to "conquer the Middle East" while pursuing nuclearisation.
