Foreign Secretary William Hague said Iran should "be more realistic about what is necessary" to reach a nuclear deal with major powers.
Hague made the comments shortly after a meeting with US Secretary of State John Kerry in Vienna.
They were joined in the Austrian city by their French and German counterparts for talks about Iran's nuclear programme.
Six months ago, a deadline to reach an agreement was set for July 20.
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The suspect, 35-year-old Willie Corey Godbolt, has been arrested and was being treated for a gunshot wound in hospital.
Mohanna Abdhou, known as Montana, was out with friends when two attackers cycled up to them and opened fire in Brent, north-west London.
The US president tweeted that Mrs May gave him "full details" of the incident during talks while he attended summits in Europe.