Speaking to reporters on Monday, Pentagon spokesman Chris Meagher said Washington was still investigating how the documents, which have circulated online, were made public, as well as the scope of the leak.
“There have been steps to take a closer look at how this type of information is distributed and to whom,” Meagher said.
“Disclosure of sensitive classified material can have tremendous implications not only for our national security, but could lead to people losing their lives.”
The documents have been steadily appearing on internet platforms in recent days, with some circulating for weeks and possibly months.
They began to receive attention in US media last week, and since then, US officials have walked a careful line in assessing their authenticity. Meagher said on Monday that US Defense Department staff were still working to make that determination.
“Photos appear to show documents similar in format to those used to provide daily updates to our senior leaders on Ukraine and Russia-related operations, as well as other intelligence updates,” he said.
Officials from several agencies have repeatedly warned that at least some of the documents appeared to have been doctored.
On Sunday, the Department of Justice opened a criminal investigation into the situation.
Many of the documents purported to include secret information on the war in Ukraine, while others appeared to show sensitive analyses of US allies, including Israel and South Korea, raising questions about surveillance.
When asked about the extent of the leak on Monday, White House National Security Council spokesman John Kirby replied: “We don’t know what’s out there.”
“We don’t know who’s responsible for this. And we don’t know if they have more that they intend to post,” he said.
“So we’re watching this and monitoring it as best we can. But the truth and the honest answer to your question is we don’t know and is that a matter of concern to us? You’re darn right it is.”
Kirby added that US President Joe Biden had been briefed on the matter late last week.
Meanwhile, State Department spokesman Vedant Patel said US officials “are engaging with allies and partners at high levels over this, including to reassure them of our commitment to safeguarding intelligence and the fidelity of securing our partnerships”.
US officials have said some documents giving battlefield casualty estimates from Ukraine appeared to have been altered to understate Russian losses.
Ukrainian presidential adviser Mykhailo Podolyak told the Reuters news agency that documents related to the war in Ukraine contained a “very large amount of fictitious information”.
On Sunday, Israeli Prime Minister Benjamin Netanyahu’s office said a document purporting to show US intelligence stating that Mossad, one of the country’s intelligence agencies, encouraged recent protests against Netanyahu’s plan to tighten controls on the judiciary was “mendacious and without any foundation whatsoever”.
One of the documents also purported to detail internal discussions among senior South Korean officials about US pressure on Seoul to supply weapons to Kyiv, and its policy of not doing so.
A South Korean presidential official said on Sunday that the country was aware of reports about the leaked documents and planned to discuss “issues raised” with Washington.
The release appears to be the most serious public leak of classified information in years, although it is believed to be much smaller in scale and scope compared to the 700,000 documents, videos and diplomatic cables that appeared on the WikiLeaks website in 2013.