Iran has voiced optimism on a nuclear deal ahead of talks in Geneva but has accused Israel of trying to sabotage them and of stoking Mideast 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.

“I go to Geneva with the determination to come out with an agreement at the end of this round,” Zarif said.

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 – a charge immediately denied by Israel.

The explosions killed at least 23 people. The attack was claimed by an al-Qaeda-linked jihadist group.

Zarif said the attack was a symptom of rising extremism which threatened to move beyond the Middle East.

Fresh talks with the United States, Britain, France, China, Russia and Germany – the so-called P5+1 – over Tehran’s nuclear program 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.

But boosting hopes on the eve of the talks was an announcement from London that David Cameron on Tuesday had become the first British premier in more than a decade to telephone an Iranian leader.

During their conversation, 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.