LONDON - Britain's spy agency chief stepped out of the shadows with an unprecedented public address Thursday, defending the need for secrecy to counter growing terror threats such as Iran's nuclear proliferation.

MI6 chief John Sawers said even though Cold War-era secrecy has been lifted and intelligence agencies were working to become more accessible keeping intelligence material secret was vital to protect people against terror attacks.

"Secrecy is not a dirty word. Secrecy is not there as a coverup," Sawers told a select group of journalists in London. "Without secrecy there would be no intelligence services, or indeed other national assets like our special forces. Our nation would be more exposed as a result."

The question of secrecy has dominated world news in the last week, after the whistleblowing group WikiLeaks published nearly 400,000 U.S. intelligence logs detailing daily carnage in Iraq since the 2003 U.S.-led invasion. In July, the same group published 77,000 secret U.S. documents on the war in Afghanistan.

Some of the leaked documents show that coalition forces handed over terror suspects to Iraqi security services even after abuse was suspected, or continued with interrogations despite visible injuries to suspects. There has been no clear mention of MI6 involvement in the logs, however.

Sawers' speech also comes as two government inquiries probe whether MI6 and other agencies were complicit in the abuse of terror suspects -- allegations that Sawers denied Thursday, adding that MI6 agents are obligated by law to stop and avoid torture.

"And we do --even though that allows terrorist activity to go ahead," he said, adding that although his agency hasn't been specifically accused of torture it has been accused of "being too close to it."

Sawers -- also known by the codename "C" as all MI6 directors have been known since the first chief Mansfield Cumming -- said progress had been made in Afghanistan and Pakistan, but new terror threats were growing in Yemen, Somalia and North Africa. The new threats come amid severe budget cuts ahead.

"We get inside terrorist organizations to see where the next threats are coming from," he said. "What we do is not seen."

He said while a "typical" terror attack would not bring down Britain, the dangers of nuclear proliferation -- as well as chemical and biological weapons -- had the potential to alter the political balance of power in the region.

"The revelations around Iran's secret enrichment site at Qom were an intelligence success," he said. "They led to diplomatic pressure on Iran intensifying, with tougher U.N. and EU sanctions which are beginning to bite. The Iranian regime must think hard about where its best interests lie."

But using intelligence poses anguished choices for agents -- especially when faced with the possibility that intelligence could be tainted by abuse or torture.

"Suppose we received credible intelligence that might save lives, here or abroad. We have a professional and moral duty to act on it," he said.

"We also have a duty to do what we can to ensure that a partner service will respect human rights. That is not always straightforward. If we hold back, and don't pass that intelligence, out of concern that a suspect terrorist may be badly treated, innocent lives may be lost that we could have saved," he said. "Sometimes there is no clear way forward."

Since the Sept. 11 terror attacks in 2001, there have been visible cultural differences in the way the U.S. and Britain handle terror threats and informing the public of intelligence information.

In a court case that drove a wedge between U.S. and British intelligence, British High Court judges last year ordered the government to release secret U.S. intelligence exchanges on the treatment of former Guantanamo Bay detainee Binyam Mohamed, who claimed British authorities knew he was beaten and had his genitals sliced with a scalpel while being interrogated in Morocco.

Sawers said the release of such information risked intelligence sharing and undermined the "Control Principle," a rule in the intelligence community that says the agency that gets the information first has the power to decide how the information is used.

Trans-Atlantic differences have also showed up during a European terror plot that was unveiled last month and is still considered active. While the United States issued a travel advisory warning citizens to be aware of potential threats in Europe, British officials downplayed the specifics of the plot.

There was also a striking difference during the trans-Atlantic bombing plot in 2006, where militants tried to down several airliners with explosives contained in soft drink bottles. Prosecutors said American officials wanted to crack the plot and round up the suspects quickly whereas British officials wanted to wait and gather more information.

"So if the control principle is not respected, the intelligence sharing dries up," he said. "That's why we have been so concerned about the possible release of intelligence material in recent (U.K.) court cases."

MI6, known as cloak-and-dagger employer of the fictional James Bond, has tried to become less secretive. It has started posting recruitment ads in Britain's media, hired press officers, and last month released its first-ever official history.

But Britain's three major intelligence agencies collectively face a 7.5 per cent cut over the next five years.

Insisted that intelligence was more important now than ever, Sawers said MI6 would be working closely with Britain's domestic spy agency, MI5, and its eavesdropping agency, GCHQ, on budget issues.

"Yes, the intelligence services have to make savings too," he said.