The Pentagon said in a statement on Thursday that the Defence Department was looking into the matter. On Friday, as the disclosures widened, department officials said they had nothing to add. But privately, officials in several national security agencies acknowledged both a rush to find the source of the leaks and a potential for what one official said could be a steady drip of classified information posted on sites.

The documents on Ukraine’s military appear as photographs of charts of anticipated weapons deliveries, troop and battalion strengths, and other plans. Pentagon officials acknowledge that they were legitimate Defence Department documents, but the copies appeared to have been altered in certain parts from their original format. The modified versions, for example, overstate US estimates of Ukrainian war dead and underestimate estimates of Russian troops killed.

Ukrainian officials and pro-war Russian bloggers suggested the leak was part of a disinformation effort by the other side, timed to influence Ukraine’s possible spring offensive to reclaim territory in the east and the south of the country.

A senior Ukrainian official said the leak appeared to be a Russian ploy to discredit a counter-offensive. And the Russian bloggers warned against trusting any of the information, which one blogger said could be the work of “Western intelligence in order to mislead our command”.

Behind closed doors, chagrined national security officials were trying to find the culprit. One official said it was likely that the documents did not come from Ukrainian officials because they did not have access to the specific plans, which bear the imprint of the offices of the Pentagon’s Joint Staff. A second official said that determining how the documents were leaked would start with identifying which officials had access to them.

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The first tranche of documents appeared to have been posted in early March on Discord, a social media chat platform popular with video gamers, according to Aric Toler, an analyst at Bellingcat, the Dutch investigative site.

In Ukraine, Lieutenant Colonel Yurii Bereza, a battalion commander with Ukraine’s national guard whose forces have fought in the country’s east in recent months, shrugged off news of the leak.

He noted that information warfare had become so intense that “we can no longer determine where is the truth and where is the lie.”

“We are at that stage of the war when the information war is sometimes even more important than the direct physical clashes at the front,” Bereza said.

A soldier in his unit, Maksym, had yet to hear the news. “We have a lot of our own problems, and with this leak I have no words,” he said angrily.

Outside experts said it was difficult to draw conclusions about who released the information and why.

Kyle Walter, the head of research at Logically, a British firm that tracks disinformation, said many prominent voices on Russian Telegram channels were calling the original, apparently unaltered photo showing Russian and Ukrainian casualties a “Western influence” operation.

“They think the actual unedited photo where it shows high Russian loss numbers and relatively low Ukrainian loss numbers is an attempt to instil poor morale in Russia and Russian forces,” Walter said.

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Jonathan Teubner, CEO of FilterLabs AI, which tracks messaging in Russia, said that while pro-Kremlin voices were saying the leak was an American or Ukrainian disinformation campaign, his lead analyst thought it could be a Russian operation meant to sow distrust between the US and Ukraine.

The doctored photo showing lower casualty numbers for Russia, and higher ones for Ukraine, than reported figures has been discussed far more frequently in Western-orientated social media than in Russian-focused platforms, Walter said.

It has been a frequent Russian disinformation tactic to alter stolen documents, including some purportedly leaked from the Ukrainian government, Walter said. But because Ukraine’s government has dismissed these documents as altered or out of context, they generally do not gain much traction, he added.

“There are a lot of examples of leaked documents being used in propaganda campaigns and specifically in terms of disinformation,” he said. But what is going on with these American documents, he added, “is still pretty unclear at the moment”.

Assistant Treasurer Stephen Jones said the Australian government was “deeply concerned” by the reports.

“We’re still getting briefings on it so I don’t really want to get ahead of that in terms of flagging a response but it puts a spotlight on what we need to do domestically, by ensuring that our cybersecurity is top notch,” he said.

“It’s why the government is committed to lifting our cybersecurity standards. We’ve got a program of work underway in that area at the moment.”

This article originally appeared in The New York Times.