Britain's foreign spy chief has reportedly warned that Iran is two years away from acquiring nuclear weapons, but analysts said he was assuming Tehran is actually developing the bomb.

MI6 boss John Sawers gave a rare public speech to civil servants in London saying that British agents had prevented Iran from producing a nuclear weapon as early as 2008, the Daily Telegraph newspaper reported Friday.

But Sawers said the threat of Iran becoming "a nuclear weapons state" was now only "two years away", said the Telegraph, citing Civil Service World, a newspaper for senior ministry officials and lawmakers which reported the event.

Should Iran finally acquire nuclear weapons, the intelligence chief warned that Israel and the United States "would face huge dangers", hinting at the increased likelihood of military action, according to the Telegraph article.

Iran since 2010 has been subject to severe international economic sanctions over its controversial nuclear programme, which Western powers believe masks an atomic weapons drive -- despite repeated denials by Tehran.

But analysts said Sawer's comments pointed to an assumption that Tehran was already developing a nuclear weapon.

"Most Western intelligence agencies believe that Iran has not made the decision to acquire a nuclear weapon, but is amassing the capability to weaponise when it decides to do so," analyst Dina Esfandiary of the London-based International Institute for Strategic Studies told AFP.

"He (Sawers) seems to be completely glossing over that," she said.

"At the moment, Iran has enough low-enriched uranium for four or five bombs if further enriched," she said, explaining that only highly-enriched uranium can be used for weapons.

Patricia Lewis, an international security expert at London's Chatham House think-tank, said she was surprised by the claims that MI6 had stopped Iran getting nuclear weapons by 2008.

"I am surprised by the date because, if you look at IAEA (International Atomic Energy Agency) documents, the general understanding is that any weapons activities that Iran had stopped in 2003," she told AFP.

She said that in Sawers's speech there were "assumptions there as to how you define a weapon... whether Iran has developed a warhead, whether it has the technology to do it."

The Telegraph quoted Sawers as saying Iran was "determinedly going down a path to master all aspects of nuclear weapons", and that "Israel and the United States would face huge dangers if Iran were to become a nuclear weapon state."

He said that without MI6's efforts, "you'd have Iran as a nuclear weapons state in 2008 rather than still being two years away in 2012," according to the article.

The report comes as the United States unleashed a fresh wave of sanctions against Iran on Thursday, ratcheting up pressure to convince Tehran to take seriously concerns about its disputed nuclear programme.

Iran has repeatedly said it will not give up its nuclear ambitions, which it insists are purely peaceful.

Lewis said one purpose of Sawers's speech appeared to be to send a message to both Iran and Israel.

"To Israel he is saying: 'they do not have them (weapons). Do not stop them, give us time. To Iran he is saying: 'we know what you're doing'. He is trying to make Iran nervous," Lewis said.

"He is also issuing a warning to the rest of us that Israel may be prepared to act if they think they have only got until 2014."