The US Justice Department on Friday lauched a probe into the possible leak of Pentagon documents on several social media platforms that appear to detail US and NATO aid to Ukraine.

But the documents may have been altered or used as part of a misinformation campaign, some US officials said.

"We have been in communication with the Department of Defense related to this matter and have begun an investigation. We decline further comment," a Justice Department spokesperson said on Friday.

Pentagon spokesperson Sabrina Singh also said internal an internal review was underway.

Speaking on the condition of anonymity, three US officials told the Reuters news agency that Russia or pro-Russian elements were likely behind the leak.

What do the documents contain?

The documents are dated from February 23, 2023, until March 1, 2023. They are also labelled as secret.

They appear to detail deliveries of weapons and other equipment going into Ukraine with more precise timelines than the US generally provides publicly. 

However, anonymous US officials told Reuters that the documents appeared to have been altered to lower the number of Russian casualties, according to their own informal assessment which is separate from the official investigation into the leak.

One document posted on social media said 16,000 to 17,500 Russian casualties since the invasion of Ukraine on February 24, 2022. US authorities have previously put given a figure of around 200,000 Russians killed and wounded. 

Ukrainian officials respond

Ukrainian officials also played down the idea of military intelligence being compromised.

"It is very important to remember that in recent decades, the Russian special services' most successful operations have been taking place in Photoshop," Andriy Yusov, a spokesperson for Ukraine's military intelligence directorate, said on Ukrainian TV.

"From a preliminary analysis of these materials, we see false, distorted figures on losses on both sides, with part of the information collected from open sources."

Ukrainian presidential advisor Mykhailo Podolyak said the documents contained a "very large amount of fictitious information" and resembled "a Russian disinformation operation to sow doubts about Ukraine's planned counter-offensive."

Separately, however, Ukrainian President Volodymyr Zelenskyy's office released a statement Friday about a meeting he had with his senior military staff, and it noted that "the participants of the meeting focused on measures to prevent the leakage of information regarding the plans of the defense forces of Ukraine." 

zc/msh (AP, Reuters, dpa)