David Cameron has become the first British prime minister in more than a decade to telephone an Iranian leader, boosting hopes for a successful nuclear deal on the eve of talks due to be held in Geneva.
During their conversation on Tuesday, Cameron and President Hassan Rouhani agreed "it was important to seize the opportunity presented" by the next round of talks in Geneva, Downing Street said in a statement.
Cameron also urged Tehran to address global concerns over its nuclear programme, the statement added, "including the need for greater transparency."
Iran has voiced optimism regarding a nuclear deal ahead of talks in Geneva, but accused Israel of trying to sabotage them and of stoking Middle East tensions, following bomb attacks on its embassy in Beirut.
"I think there is every possibility for success," Iranian Foreign Minister Mohammad Javad Zarif, Iran's top nuclear negotiator, said after meeting his Italian counterpart Emma Bonino in Rome.
But he said Israel was trying to undermine the talks, after an Iranian foreign ministry spokeswoman accused Tel Aviv of being behind the attacks on Iran's embassy. The charge was immediately denied by Israel.
The explosions killed at least 23 people and the attack was claimed by an Al-Qaeda-linked group.
Zarif did not repeat the claim against Israel but said: "We have reason to be suspicious of every move they make."
Referring to the nuclear negotiations, he added: "They have been trying so hard to torpedo the process."
Earlier on Tuesday, Zarif sought to reassure a foreign audience by posting his first-ever video statement on YouTube, which is blocked in Iran.
"For us Iranians nuclear energy is not about joining (a) club or threatening others," he said in English in the video.
Fresh talks with the United States, Britain, France, China, Russia plus Germany, the so-called P5+1, over Tehran's nuclear programme are due to start in Geneva on Wednesday.
International powers aim to convince Iran to roll back work that they suspect is masking a military nuclear drive. In exchange, they are offering relief from sanctions.
US State Department spokeswoman Jen Psaki said the talks were "the best chance we've had in years, if not decades, to move this process forward."
But US President Barack Obama struck a more cautious tone, refusing to say a deal was within reach.
"I don't know if we will be able to able to close a deal this week or next week," Obama said at a Wall Street Journal CEO forum, insisting any relief from crippling international sanctions that Iran could expect under an interim pact was highly limited.