Britain's top spy on Thursday gave his first ever public speech and claimed that it was the work of secret agents that revealed Iran was enriching nuclear fuel at a secret site.

Britain's Secret Intelligence Service (SIS) chief John Sawers addresses a live televised gathering of academics, officials and editors in London October 28, 2010. [Xinhua]

Sir John Sawers is the head of MI6, Britain's secret intelligence service, which runs networks of spies in foreign countries.

The head of MI6, also known as the Secret Intelligence Service (SIS), has usually not been named in the past, and is known throughout government just as 'C.' Sawers' speech is the first time that the head of MI6 has spoken publicly about the organization.

Sawers, speaking at a Society of Editors' meeting in London, said: "The revelations around Iran's secret enrichment site at Qom were an intelligence success. They led to diplomatic pressure on Iran intensifying, with tougher UN and EU sanctions which are beginning to bite."

He said that the dangers of the proliferation of nuclear, chemical, and biological weapons were far-reaching, and claimed they could "alter the whole balance of power in a region."

He added: "States seeking to build nuclear weapons against their international legal obligations are obsessively secretive about it. SIS's role is to find out what these states are doing and planning, and identify ways to slow down their access to vital materials and technology."

Western countries have called on Tehran to halt its sensitive nuclear program, but the country has continued with it, insisting that its nuclear activities are for civilian purposes only.

The speech by Sawers follows the first ever public speeches, over the past few weeks, by Iain Lobban, the director of the Government Communication HQ which is the base for Britain's electronic eavesdropping operations, and by Jonathan Evans, the head of MI5, which is responsible for domestic security.