By Mark Hosenball

WASHINGTON (Reuters) - British and U.S. intelligence officials say they are worried about a "doomsday" cache of highly classified, heavily encrypted material they believe former National Security Agency contractor Edward Snowden has stored on a data cloud.

The cache contains documents generated by the NSA and other agencies and includes names of U.S. and allied intelligence personnel, seven current and former U.S. officials and other sources briefed on the matter said.

The data is protected with sophisticated encryption, and multiple passwords are needed to open it, said two of the sources, who like the others spoke on condition of anonymity to discuss intelligence matters.

The passwords are in the possession of at least three different people and are valid for only a brief time window each day, they said. The identities of persons who might have the passwords are unknown.

Spokespeople for both NSA and the U.S. Office of the Director of National Intelligence declined to comment.

One source described the cache of still unpublished material as Snowden's "insurance policy" against arrest or physical harm.

U.S. officials and other sources said only a small proportion of the classified material Snowden downloaded during stints as a contract systems administrator for NSA has been made public. Some Obama Administration officials have said privately that Snowden downloaded enough material to fuel two more years of news stories.

"The worst is yet to come," said one former U.S. official who follows the investigation closely.

Snowden, who is believed to have downloaded between 50,000 and 200,000 classified NSA and British government documents, is living in Russia under temporary asylum, where he fled after traveling to Hong Kong. He has been charged in the United States under the Espionage Act.

Cryptome, a website which started publishing leaked secret documents years before the group WikiLeaks or Snowden surfaced, estimated that the total number of Snowden documents made public so far is over 500.

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Given Snowden's presence in Moscow, and the low likelihood that he will return to the United States anytime soon, U.S. and British authorities say they are focused more on dealing with the consequences of the material he has released than trying to apprehend him.

It is unclear whether U.S. or allied intelligence agencies - or those of adversary services such as Russia's and China's -

know where the material is stored and, if so, have tried to unlock it.

One former senior U.S. official said that the Chinese and Russians have cryptographers skilled enough to open the cache if they find it.

Snowden's revelations of government secrets have brought to light extensive and previously unknown surveillance of phone, email and social media communications by the NSA and allied agencies. That has sparked several diplomatic rows between Washington and its allies, along with civil liberties debates in Europe, the United States and elsewhere.

Among the material which Snowden acquired from classified government computer servers, but which has not been published by media outlets known to have had access to it, are documents containing names and resumes of employees working for NSA's British counterpart, the Government Communications Headquarters (GCHQ), sources familiar with the matter said.

The sources said Snowden started downloading some of it from a classified GCHQ website, known as GC-Wiki, when he was employed by Dell and assigned to NSA in 2012.

Snowden made a calculated decision to move from Dell Inc to another NSA contractor, Booz Allen Hamilton, because he would have wide-ranging access to NSA data at the latter firm, one source with knowledge of the matter said.

"EXTREME PRECAUTIONS"

Glenn Greenwald, who met with Snowden in Hong Kong and was among the first to report on the leaked documents for the Guardian newspaper, said the former NSA contractor had "taken extreme precautions to make sure many different people around the world have these archives to insure the stories will inevitably be published."

"If anything happens at all to Edward Snowden, he has arranged for them to get access to the full archives," Greenwald said in a June interview with the Daily Beast website. He added: "I don't know for sure whether has more documents than the ones he has given me... I believe he does."

In an email exchange with Reuters, Greenwald, who has said he remains in contact with Snowden, affirmed his statements about Snowden's "precautions" but said he had nothing to add.

Officials believe that the "doomsday" cache is stored and encrypted separately from any material that Snowden has provided to media outlets.

Conservative British politicians, including Louise Mensch, a former member of parliament, have accused the Guardian, one of two media outlets to first publish stories based on Snowden's leaks, of "trafficking of GCHQ agents' names abroad."

No names of British intelligence personnel have been published by any media outlet. After U.K. officials informed the Guardian it could face legal action, the newspaper disclosed it had destroyed computers containing Snowden material on GCHQ, but had provided copies of the data to the New York Times and the U.S. nonprofit group ProPublica.

Sources familiar with unpublished material Snowden downloaded said it also contains information about the CIA - possibly including personnel names - as well as other U.S. spy agencies such as the National Reconnaissance Office and National Geospatial-Intelligence Agency, which operate U.S. image-producing satellites and analyze their data.

U.S. security officials have indicated in briefings they do not know what, if any, of the material is still in Snowden's personal possession. Snowden himself has been quoted as saying he took no such materials with him to Russia.

(Editing by Warren Strobel and Tim Dobbyn)