Secret military documents leaked in recent days appear to have been altered to lower the number of casualties suffered by Russian forces, US officials said, adding their assessments were informal and separate from the investigation into the leak itself.
It is believed that Russia or pro-Russian elements are likely behind the leak of several classified US military documents posted on social media that offer a partial, month-old snapshot of the war in Ukraine, three US officials told Reuters.
The US Justice Department said separately it was probing the leak.
The officials spoke on condition of anonymity given the sensitivity of the matter and declined to discuss the documents in any detail.
The Kremlin and Russia's embassy in Washington did not immediately respond to requests for comment.
An initial batch of documents circulated on sites including Twitter and Telegram, dated 1 March and bearing markings showing them classified as "Secret" and "Top Secret."
Yesterday, an additional batch appearing to detail US national security secrets pertaining to areas including Ukraine, the Middle East and China also surfaced on social media, the New York Times reported.
Reuters was not able to verify the authenticity of the documents.
The US Justice Department said late yesterday it was in touch with the Defense Department and began a probe into the leak. It declined further comment.
A leak of such sensitive documents is highly unusual.
"We are aware of the reports of social media posts and the Department is reviewing the matter," Pentagon spokesperson Sabrina Singh said.
A CIA spokesperson said the agency was also aware of the posts and was looking into the claims.
One document posted on social media said 16,000 to 17,500 Russian forces had been killed since Russia's invasion of Ukraine.
The US believes the actual figure is much higher, at around 200,000 Russians killed and wounded, officials have said.
A Ukrainian presidential official that the leak contained a "very large amount of fictitious information" and looked like a Russian disinformation operation to sow doubts about Ukraine's planned counter-offensive.
"These are just standard elements of operational games by Russian intelligence. And nothing more," Mykhailo Podolyak said in a written statement.
Meanwhile, a missile fired from Ukraine was shot down over the Black Sea town of Feodosia in Russian-controlled Crimea, the Russian head of Crimea's administration said.
"A missile launched from Ukraine was shot down over Feodosia," Sergei Aksyonov said on Telegram.
Located in the east of Crimea, Feodosia is almost 300kms from the nearest Ukrainian-held area.
Kyiv, which had no immediate comment, is not publicly known to possess missiles with that range.
Russia's TASS news agency quoted an adviser to Aksyonov, Oleg Kryuchkov, as saying that debris had fallen in a Crimean town but no damage or casualties had been reported.
Reuters could not immediately verify the battlefield reports.
Read more on Russia's invasion of Ukraine
In Russia hundreds of supporters including Yevgeny Prigozhin, the chief of mercenary outfit Wagner, turned out for the funeral of a high-profile Russian military blogger, 40-year-old Vladlen Tatarsky, who was killed in a bombing attack last week.
An explosion destroyed a café in Russia's second city Saint Petersburg, killing Mr Tatarsky, who was known for his staunch anti-Ukraine stance.
A 26-year-old Russian woman, Darya Trepova, was detained and charged with terrorism.
Investigators said Ms Trepova had brought a statuette rigged with explosives to a café in Saint Petersburg and handed it over to the blogger, whose real name was Maxim Fomin.
Mourners, some carrying flowers, gathered at the prestigious Troyekurovskoye cemetery in western Moscow amid beefed-up police presence. Some supporters wore clothes with the letters Z and V, which are symbols of Moscow's assault on Ukraine.
Mr Tatarsky's awards were placed on velvet cushions near his casket. Among them was the Order of Courage, one of the country's top awards that President Vladimir Putin posthumously bestowed on Mr Tatarsky for his "bravery."
Mr Tatarsky, who hailed from the eastern Ukrainian region of Donetsk, fought alongside pro-Kremlin separatists and then became a popular blogger with half a million followers on social media.
At a Kremlin ceremony announcing the annexation of four Ukrainian regions last September, Mr Tatarsky recorded himself saying: "We will defeat everyone. We will kill everyone. We will rob everyone as necessary. Just as we like it."
In a statement released by his spokespeople he was praised as a "voice (that) that will always live and speak only the truth".
Russian authorities claim without any evidence that supporters of jailed Kremlin critic Alexei Navalny helped Ukrainian authorities carry out the bombing attack.
Mr Putin this week accused Western security services of having helped Kyiv stage "terror attacks" in Russia.
It has been announced that Ukrainian minister will visit India tomorrow in the first face-to-face talks between the two countries since Russia's invasion of its European neighbour last year.
India imports much of its military hardware from Russia and is walking a delicate balancing act between its increased security cooperation with Western countries and its reliance on Russia for defence and oil imports.
Its longstanding security ties with Russia have put New Delhi in an awkward diplomatic position, and while it has called for an end to hostilities in Ukraine, it has refused to condemn the invasion.
Emine Dzhaparova, Kyiv's first deputy foreign minister, will "exchange views on the current situation in Ukraine" with a senior member of India's foreign ministry, according to an Indian government statement.
India shares "warm and friendly relations and multifaceted cooperation" with Ukraine and the visit will be an occasion to "further mutual understanding and interests", the foreign ministry statement added.
There was no indication that Ms Dzhaparova would meet with Prime Minister Narendra Modi or ministers from his government.
A report in The Hindu newspaper said Ms Dzhaparova would meet with media and think-tanks in New Delhi in an effort to build support for Ukraine.
She was also likely to invite Modi to visit Kyiv, it said.
While India has stopped short of publicly denouncing the invasion, Mr Modi told Russian President Vladimir Putin last year that this was "not a time for war" in comments seen as a rebuke to Moscow.
But India is also snapping up discounted crude oil from Russia, resisting Western pressure to freeze out Moscow.
It has opted to strengthen trade ties with its long-standing ally, with the added benefits of tempering inflation while saving public money.
Meanwhile Russia's Gazprom said it would ship 37.2m cubic metres of gas to Europe via Ukraine today.
