The US declared its determination to "turn over every rock" to find out who was behind it, the UK issued a warning to readers.

Vụ rò rỉ tài liệu mật: Mỹ tuyên bố quyết 'lật từng tảng đá' để tìm cho ra kẻ đứng sau, Anh phát cảnh báo tới người đọc. (Nguồn: Reuters)

US Defense Secretary Lloyd Austin speaks at a press conference on April 11 about the leak of classified documents. (Source: Reuters)

Secretary Austin was the first senior US official to comment on the incident. Speaking at a press conference, the head of the Pentagon said that the agency had discovered the documents posted on February 28 and March 1, but was not sure whether any other documents had circulated online before.

“These are things we will find out,” Mr Austin said. “ We take this very seriously and will continue to investigate and turn over every stone until we find the source and the extent of the incident.”

Previously, on April 10, Assistant Secretary Austin Chris Meagher said that the leak of sensitive US documents posed a "very serious" risk to national security, and assessed that the documents circulating online were likely to lead to "the spread of misinformation".

Meanwhile, Director of the US Central Intelligence Agency William Burns called the leak "extremely regrettable", showing the urgency of tightening information security steps.

US national security agencies and the Justice Department are stepping up their investigation into the leak, while assessing the damage to national security and relations with allies and other partner countries, including Ukraine.

Security agencies are reviewing how sensitive information is shared within the government and responding to diplomatic fallout from the release of sensitive documents from unknown sources, US officials said.

Also on April 11, US Secretary of State Antony Blinken and Secretary Austin spoke with their Ukrainian counterparts Dmytro Kuleba and Oleksiy Reznikov to reassure allies about the leak.

Speaking at a press conference, according to Mr. Blinken, Washington has maintained high-level contact with partner and allied countries in recent days, reaffirming its commitment to ensuring intelligence security.

On the same day, the British Ministry of Defense announced that the contents of what are believed to be secret US documents, which are currently being widely circulated, are "seriously inaccurate" and warned readers to be cautious when receiving this seemingly valuable information.

Australia, an ally in Oceania, has also spoken out about the incident. The country’s Chief of Defence Force, Angus Campbell, described the possible leak of US intelligence as a “serious” incident and said Washington was working with its partners to understand the consequences.

"Maintaining information security is critical to the development of national capabilities as well as trust and confidence among allies and partners," Campbell said in a speech at the Lowy Institute.

On the side of South Korea - America's ally in Northeast Asia - Deputy National Security Advisor Kim Tae-hyo said that the country's Defense Minister had a phone call with his US counterpart, in which the two sides agreed that most of the information contained in the documents leaked online was "fake".

Meanwhile, Mikhail Podolyak, advisor to the Chief of Staff of the Ukrainian President, said that the leak of confidential documents about Kiev's counterattack would not affect the Eastern European country's relations with Western countries or any of Kiev's plans.

Since last week, information about leaked US classified documents on social media has exploded in the media, with mentions of alleged military vulnerabilities in Ukraine, about US allies such as Israel, South Korea, or about China and the Middle East.

This could be the most damaging leak of information to the US government since the release of thousands of documents on WikiLeaks in 2013.

Reuters has reviewed more than 50 documents, marked "Confidential" and "Top Secret," but has not been able to independently verify the authenticity of the leaked documents.