John Kerry to press for 'critical choices' in second meeting with Iran's Javad Zarif

US Secretary of State John Kerry will press his Iranian counterpart Mohammad Javad Zarif to make "critical choices" in a second straight day of talks over Tehran's nuclear programme on Monday, a US official said.

Kerry and Zarif met for two hours on Sunday on the sidelines of a meeting between Iran and six major negotiating powers - the US, Britain, France, Germany, Russia and China - as a July 20 deadline for a deal with Tehran looms.

Kerry would "gauge Iran's willingness to make the critical choices it needs to make", a senior State Department official said. "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 said.

Kerry said on Sunday there were still substantial gaps with Iran on how to reduce its nuclear fuel-making capacity, suggesting that the talks may go beyond July 20 unless progress is made this week.

Earlier, a senior US official said Iran was sticking to "unworkable and inadequate" positions.

Iran says 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.