US Secretary of State John Kerry and his Iranian counterpart Javad Zarif have met for a second day of talks in Vienna, as they try to make progress on reaching a nuclear arms control deal before a June 20 deadline.
Kerry would "gauge Iran's willingness to make the critical choices it needs to make," a senior State Department official said on Monday ahead of the meeting in the Austrian capital.
"The Secretary will take the time necessary to have that discussion, and that's why they will be meeting again today, to see if progress can be made," the official told the Reuters news agency.
On Sunday, Kerry met with Zarif but the two sides failed to agree on a deal, although there were some indications that the talks would be extended past the deadline.
Kerry said there were still substantial gaps with Iran on how to reduce its nuclear fuel-making capacity.
A senior US official had also said that Iran was sticking to "unworkable and inadequate" positions.
Iran maintains it is enriching uranium for peaceful energy purposes only and wants Western sanctions lifted. But a history of hiding sensitive nuclear work from UN inspectors has kept international suspicions high.
After the meeting on Sunday, Zarif said that "our team is ready to work with full speed during the seven remaining days in order to reach a comprehensive deal that can be acceptable for both sides."
Other countries represented in the talks are Britain, France, Germany, China, and Russia.
Frank Walter Steinmeier, the German foreign minister, said the ball was in "Iran's court now".
"It is now up to Iran to decide whether they are looking for a way to cooperate with the international community or if they want to remain in isolation," Steinmeier said on Sunday.
"I hope the days until July 20th will be enough to allow Tehran to reflect and at the end pave the way for an agreement. The ball is in Iran's court now."
The West wants Iran to significantly scale back its nuclear programme to address its concerns of proliferation risks, while Iran insists its nuclear rights are inalienable.
An extension of the talks would give more time to negotiate a deal that would limit the scope of such programmes in exchange for a full lifting of nuclear-related sanctions imposed on Iran.