Edward Snowden
The signature (L) on the letter of fugitive former U.S. spy agency contractor Edward Snowden. (Reuters)

The US National Security Agency (NSA) has revealed that whistleblower Edward Snowden leaked up to 200,000 classified documents to the media, suggesting the agency's reputation could be damaged by further revelations over the coming weeks.

"Snowden has shared somewhere between 50 and 200,000 documents with reporters. These will continue to come out," NSA director general Keith Alexander said.

Alexander was responding to a question about the steps that US authorities were taking to stop the former NSA contractor from leaking additional information to the media.

The NSA chief also noted it is "very hard" for his agency to prevent such leaks from Snowden.

"They're being put out in a way that does the maximum damage to NSA and our nation... And it's hurting our industry. I think it's wrong. I don't know how to stop it," he said.

"But I'll tell you, this increases the probability that a terrorist attack will get through. I think it's absolutely wrong. When we look back on this, people are going to see that and understand that and say what they did was wrong. Until then, we're at their mercy.

"They're putting them out, one or two a week, to cause the maximum problem. They get it wrong."

The revelations of spying by the secret agency that have emerged thus far have created a major headache for the Obama administration.

Spying Scandal

Snowden, a former contractor at the NSA, had earlier leaked top secret documents about a global programme of surveillance by Washington. He is currently in exile in Russia.

The documents revealed that the NSA has been tapping telephone conversations and spying on the internet activity of citizens, leaders, bureaucrats, businesses and government agencies.

Following the revelations, the NSA's surveillance practices have drawn sharp criticism within the US and around the world.

According to the leaked documents, prominent leaders spied on by the NSA include Brazil President Dilma Rousseff and German Chancellor Angela Merkel. In addition to criticising the Obama administration severely, the countries presented a resolution in the UN to strengthen international regulations on data protection.