NEW YORK -- The Pentagon is assessing the damage after the leak of highly sensitive intelligence documents.
The Justice Department is now launching a criminal investigation into the leak.
The documents, some marked as top-secret, appear to reveal information about U.S. allies, including South Korea and Israel.
They also appear to include details about the condition of Ukraine's air-defense systems, crucial for the war against Russia.
Experts say the big concern is partners like Ukraine may now think twice before sharing secrets with the U.S. again.
The Justice Department is scrambling to find the source of what appears to be a major leak of U.S. intelligence documents.
"We are talking about documents that are labeled 'top secret,' that as high as you can go when it comes to U.S. intelligence," said Luis Martinez, ABC News Senior Pentagon Reporter.
ABC News has reviewed more than 30 pages of the highly classified material posted online, containing what appears to be U.S. intelligence about the war in Ukraine, Russia, Iran's nuclear program, North Korea's missile program, China, and other countries.
The files also reveal the U.S. was not only spying on adversaries but also allies.
"There is potential to damage the relationship with some of our allies because some of this information could have been gleaned if the U.S. was spying on senior leaders," Martinez said.
A headline in the Washington Post said the files provide details about the near-downing of a British spy plane, near Ukraine, last year.
"When do you put this into a newspaper or a television broadcast, but we have reached a point where these things proliferated through telegram, Twitter," said Dan Lamonthe, The Washington Post.
Some documents appear to be from March 1 showing how the U.S. and NATO were helping Ukraine, details about casualty numbers, training schedules, and weapons deliveries, and a map of the key battle raging in a city in eastern Ukraine.
In at least one instance, the documents appear to be altered to favor Russia's point of view.
In a statement Sunday night, the Pentagon said its "...highest priority is the defense of our nation and our national security. We have referred this matter to the Department of Justice, which has opened a criminal investigation."
Analysts say this could be the worst intelligence leak since Edward Snowden.
But in this case, the documents appear to be more timely.
"The fact that they were posted so close to when they were published, that makes it really scary for operational security," Martinez said.
At least one U.S. ally, Israel, has denied a specific incident described in the documents.