Iranian nuclear talks will go past their June 30 deadline, according to a senior US official.
Iran's foreign minister Mohammed Javad Zarif is leaving the talks in Vienna, Austria, to fly back to Tehran where he will receive further instructions on how to proceed.
The seven countries taking part in the talks are yet to reach a deal on how much access Tehran should give to UN experts monitoring Iran's compliance with any agreement.
The United States insists on more intrusive access than Iran is ready to give.
US Secretary of State John Kerry met in Vienna for their third encounter since Saturday. German foreign minister Frank-Walter Steinmeier and French foreign minister Laurent Fabius are also in Vienna, and their Russian and British counterparts were to join later. China was sending a deputy foreign minister in a building diplomatic effort to wrap up the negotiations.
"Given the dates, and that we have some work to do ... the parties are planning to remain in Vienna beyond June 30 to continue working," said the official.
Asked about the chances for a deal, Federica Mogherini, the European Union's top diplomat, told reporters: "It's going to be tough ... but not impossible."
Mr Steinmeier told German media earlier: "I am convinced that if there is no agreement, everyone loses.
"Iran would remain isolated. A new arms race in a region that is already riven by conflict could be the dramatic consequence."
Israeli Prime Minister Benjamin Netanyahu - an opponent of the talks - said: "It is still not too late to go back and insist on demands that will genuinely deny Iran the ability to arm itself with nuclear weapons."
The goal of the talks involving Iran and the US, Britain, China, France, Germany and Russia is a deal that would restrict Tehran's capacity to make nuclear weapons in exchange for sanctions relief. Iran insists it does not want such arms but is bargaining in exchange for sanctions relief.
On Saturday, diplomats told The Associated Press that Iran was considering a US-backed plan for it to send enriched uranium to another country for sale as reactor fuel, a step that would resolve one of several outstanding issues.