A nuclear deal with Iran is still possible if the country shows more flexibility, the UK said on Tuesday.
After talks with his American counterpart John Kerry in London, UK Foreign Secretary Philip Hammond described this week's nuclear talks in Vienna as extremely tough.
However, he said "I believe a deal can be done," calling on Tehran to "show more flexibility" if the talks are to succeed.
On Tuesday, negotiations between Iranians and six world powers (known as 5+1) entered a final phase ahead of the 24 November deadline.
If a deal is agreed upon, it would be seen as historic as it may defuse more than a decade of tension between western powers and the Islamic republic over its nuclear programme.
Asked about his expectations for the Vienna negotiations, Kerry declined to make any predictions. However, he said this is a "critical week," and expressed his "hope to get there."
The six powers – the US, Britain, France, Russia, Germany and China – want Iran to scale down its capacity to refine uranium so that it will take much longer to produce materials needed for building a bomb.
Iran has confirmed it is enriching uranium only to make fuel for nuclear power plants and medical and civilian purposes.