Nov. 26, 2010 -- With the website WikiLeaks set to release a new trove of sensitive information, the U.S. government is already bracing for the worldwide fallout, preemptively warning allies in the hope of lessening the blow after classified documents go public.

WikiLeaks and its controversial founder, Julian Assange, are reportedly prepared to publish a cache of information including hundreds of thousands of diplomatic cables that could embarrass the U.S. government as well as other governments around the world.

WikiLeaks said its next release will be seven times the size of its most recent leak, in October, which contained about 400,000 Pentagon documents about the war in Iraq. WikiLeaks also published roughly 70,000 documents in July about the war in Afghanistan.

Senior U.S. officials warn that the next round of WikiLeaks documents would be considerably more damaging than the two previous WikiLeaks document dumps.

"This is outrageous and dangerous," a senior U.S. official told ABC News. "This puts at risk the ability of the United States to conduct foreign policy. Period. End of paragraph."

The chairman of the Joint Chiefs of Staff also weighed in today, telling CNN he hoped these kinds of leaks will eventually be plugged.

"I would hope that those who are responsible for this would, at some point in time, think about the responsibility that they have for lives that they're exposing and the potential that's there and stop leaking this information," Adm. Mike Mullen said in an interview set to air Sunday.

Watch 'World News With Diane Sawyer' for more on this story tonight on ABC

Although the State Department said it did not know specifically what could be released, the scope of the documents goes far beyond Iraq and Afghanistan, essentially detailing day-to-day operation of U.S. foreign policy, including summaries of confidential discussions with foreign officials and intelligence sources, and dissidents and opposition figures.

The big worry among U.S. authorities is that the documents would reveal names and detailed discussions with individuals who expected that their conversations with U.S. officials would be kept confidential. In the case of intelligence sources and dissidents in oppressive countries, this could put lives of U.S. sources at risk, authorities say.

"It's very worrisome. We don't want to see people taken out and shot on the streets of Kabul or Baghdad because they've worked with the United States," said Michael O'Hanlon, an expert on defense policy with the Brookings Institution.

Bradley Manning Believed to Be Source of New Information

As with the previous WikiLeaks documents, the source this time is believed to be Army Intelligence Spc. Bradley Manning, who was arrested for leaking the classified information in July.

The 22-year-old Army specialist had access to hundreds of thousands of diplomatic cables through military computers, thanks to intelligence changes put in place after the 9/11 attacks. Those changes intended to improve intelligence sharing among different agencies and departments.

In an effort to limit the damage, the administration is working with news organizations to whom WikiLeaks has given the documents to redact sensitive names, but there is no guarantee that WikiLeaks won't simply publish unredacted documents.

The U.S. government has also already started to notify allies and U.S. lawmakers about potentially damaging information, which it said could endanger lives and harm national security.

"We are in touch with our posts around the world. They have begun the process of notifying governments that the release of documents is possible in the near future," State Department spokesman P.J. Crowley told reporters at a press briefing Wednesday.

U.K. officials confirmed today that U.S. authorities had already briefed them about the possible WikiLeaks release, and the U.S. government had also reportedly reached out to Israel, Australia and Canada, among other countries.

State Dept: Leak 'Could Create Tension'

Crowley did not provide specifics on the content of the documents, although he did say they contained classified discussions, analysis and perspectives that could harm diplomatic relations if released. According to The Associated Press, some of the information is so serious it could result in American diplomats getting expelled by foreign governments.

"This back and forth between government, the government of the United States and governments around the world, it is diplomacy in action," Crowley said. "These revelations are harmful to the United States and our interest. They are going to create tension between our diplomats and our friends around the world."

On its Twitter page in recent days, WikiLeaks appears to be taunting U.S. authorities.

"The Pentagon is hyperventilating again over fears of being held to account," the organization wrote on Nov. 23.

"Should WikiLeaks expose the world's secret diplomatic backroom dealings? Put it to the vote!" it posted in another tweet Thursday, providing a link to an online poll.

U.S. Ambassador Calls WikiLeaks 'Absolutely Awful,' Could Harm U.S. Efforts

In Baghdad today, U.S. Ambassador to Iraq James Jeffrey confirmed that the WikiLeaks documents could do serious harm to U.S. diplomatic efforts.

"WikiLeaks are an absolutely awful impediment to my business, which is to be able to have discussions in confidence with people," Jeffrey told reporters at a briefing, according to news agency AFP. "I do not understand the motivation for releasing these documents. They will not help. They will simply hurt our ability to do our work here."

Click here to return to the World News page.