"We don't know who is responsible," said US spokesman John Kirby. "We don't know if they have more [documents] to post online."
ADVERTISEMENTThe leak of highly classified US documents is a "very serious" security risk, the Pentagon's top spokesman said on Monday.
Washington is taking steps to mitigate the damage from the disclosure, which revealed sensitive information about the conflict in Ukraine, such as the situation on the front and Kyiv's crucial anti-aircraft defences.
"We don't know who is responsible," said National Security Council spokesman John Kirby. "We don't know if they have more [documents] to post. Is this a concern for us? Absolutely."
The damaging leak has raised eyebrows among US allies.
Intelligence involving other nations was revealed, with some documents mentioning that Israel's secret service Mossad would defend demonstrations inside the country against controversial judicial reform.
Many are marked "secret" or "top secret", though Euronews cannot confirm their authenticity.
US Defence Secretary Lloyd Austin has reportedly spoken to US partners, held daily meetings to assess the damage and set up a group to figure out what was lost and who had access to the information.
"...I've been convening senior department leaders daily on our response. And I have directed an urgent cross-department effort. And we've referred the matter to the Department of Justice, which has opened a criminal investigation" he said.
"Now, I can't say much more while the Justice Department's investigation is ongoing. But we take this very seriously and we will continue to work closely with our outstanding allies and partners. And nothing will ever stop us from keeping America secure" he added.
Such documents are circulating online is "a very serious risk to national security and has the potential to fuel disinformation", US Department of Defence spokesman Chris Meagher told reporters on Monday.
Part of the intelligence brief revealed that Russian spies were caught boasting about convincing the United Arab Emirates (UAE) to "work together against US and UK intelligence agencies".
The Emirati government has dismissed these allegations as "categorically false", though concerns have grown in Washington that the UAE has helped Russia circumvent sanctions over the Ukraine invasion.
A criminal investigation has been opened by the US Justice Department, with President Joe Biden said to be following the situation closely.
Images of classified documents have circulated on Twitter, Telegram, Discord and other platforms in recent days, although some may have done the rounds for weeks before attracting media attention.
Many of them are no longer available on sites where they originally appeared, with US officials scrambling to have them purged.
US officials are reaching out to concerned governments "to reassure them of our commitment to protecting intelligence and our ability to secure our partnerships," said US diplomacy spokesman Vedant Patel.
Defence Ministry spokesman Meagher said a team was working to determine whether the documents were genuine.
"Photos appear to show documents in a format similar to that used to provide daily updates to our senior Ukraine and Russia-related operations, as well as other intelligence updates," he said.
ADVERTISEMENTBut added: "Some seem to have been edited."
At least one of the documents appears to have been altered to suggest that Ukraine suffered greater losses than Russia when the original said the opposite was the case.
The consequences of these leaks could be significant, even deadly for US intelligence sources.
“The release of classified and sensitive information can not only have enormous consequences for our national security but also lead to death,” warned Meagher.