Talks with Iran on its nuclear programme are intensifying as the deadline for a deal at the end of the month draws close.
The talks, attended by Iran and the P5+1 group, the US, China, Russia, France, the UK and Germany, are intended to seek a deal to prevent Iran from seeking a nuclear agreement.
The US Secretary of State John Kerry and his German and French counterparts have cancelled their travel plans for the last few days of the talks, in order to push for a resolution ahead of the March 31 deadline.
Iran, which denies it is pursuing nuclear weapons, is hoping to achieve the lifting of international sanctions through a deal.
US officials said all parties, including Iran, had agreed "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, the BBC said.
"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.
Iranian Foreign Minister Mohammad Javad Zarif said 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.