The document is part of a trove of highly sensitive material that has been posted online, sparking a US criminal investigation into a breach the Pentagon says poses a "very serious" risk to national security.

Ukraine is expected to launch an attack on invading Russian troops in the spring -- its first major military push of the year.

But the top secret document says tough Russian defenses and "enduring Ukrainian deficiencies in training and munitions supplies probably will strain progress and exacerbate casualties during the offensive," the Post reported.

Another document says Egypt's President Abdel Fattah al-Sisi ordered the production of 40,000 rockets for shipment to Russia, telling officials to keep it secret to "avoid problems with the West," the Post reported in a separate article.

But Washington has pushed back against that report. "We have no indication that any such plan has ever been executed," a senior administration official said.

"Egypt is a close partner and we are regularly engaged with its leadership on a host of regional and global issues," the official said.

Dozens of photographs of documents have been found on Twitter, Telegram, Discord and other sites in recent days, though some may have circulated online for weeks, if not months, before they began to receive media attention.

In addition to information on Ukraine, the documents also include sensitive analyses of US allies, whom American officials are now seeking to reassure after the security breach.

Many of the documents are no longer available on the sites where they first appeared, and the United States is reportedly working to have them removed.

The fallout from the apparent leak could be significant -- even deadly -- potentially putting US intelligence sources at risk, while giving the country's foes valuable information.