As United States media has increasingly reported on the apparent leak, officials have walked a careful line in their response, with Pentagon spokesman Chris Meagher saying that the circulating photos “appear to show documents similar in format” to those provided to senior military leaders, but stressing that defence department staff were still assessing their authenticity.

Several officials have also warned that at least some of the documents appear to be doctored, fuelling concerns that they could feed misinformation campaigns. On Monday, Meagher said the documents could pose “a very serious risk to national security”.

While further details have continued to emerge, it is clear the situation has sent shock waves through the US administration, as it seeks to contain and assess the scope of any data breach, while roiling allies.

Here’s what you need to know.

Where did the documents first emerge?

According to the investigative website Bellingcat, the documents appear to have first emerged in photos posted on Discord, an online social media platform that is popular with gamers.

The photos show creased documents placed on top of magazines and other household objects. Former officials who reviewed the photos told the New York Times they appeared to have been folded, possibly to be taken from a secure location in a pocket.

Bellingcat further traced the leak to a now-defunct Discord server. Their research suggested that the documents had already appeared on social media in March, when some were dated. The open-source investigative website also said it had viewed evidence some of the documents were posted as far back as January on Discord.

The documents bear classified markings, with some labelled “top secret”, the highest level of classification, and appear to be briefing slides prepared by the US military’s chief of staff.

Some also contain the marking NOFORN, or “Not Releasable to Foreign Nationals”, meaning they cannot be shared with foreign intelligence agencies, including Five Eyes, the collection of spy agencies in the US, Canada, the United Kingdom, Australia and New Zealand, according to the New York Times.

The images of the documents spread from Discord to the online imageboard 4chan, before emerging on more mainstream social media platforms, according to Bellingcat.

Who may have leaked the documents?

The US Department of Justice has launched an investigation into the incident, but amid much speculation, no official information has been released on a possible leaker.

The NOFORN classification and geographical breadth of information contained in the documents have led some to speculate the leaker is American, although two US officials told the Reuters news agency that authorities had not ruled out the possibility the documents may have been altered to hide their origin.

One official told Reuters that investigators were looking at four or five theories, from a disgruntled employee to an insider threat who actively wanted to undermine US national security interests.

It was also unclear how the documents came into the possession of the person who leaked them.

Are the documents real?

On Sunday, the Pentagon said at least some of the documents “appear to contain sensitive and highly classified material”, but the defence department has skirted categorical claims over the authenticity of the documents, while repeatedly stressing that at least some have been doctored.

While US officials have remained tight-lipped over which parts may have been altered, one of the papers shows estimates of Russian military personnel deaths in Ukraine to be significantly lower than numbers confirmed by the US. The numbers of Ukrainian military deaths conversely appear much higher than official figures.

On Monday, the Pentagon’s Meagher told reporters “there have been steps to take a closer look at how this type of information is distributed and to whom”.

“Disclosure of sensitive classified material can have tremendous implications not only for our national security, but could lead to people losing their lives,” he said.

Vedant Patel, a spokesperson for the US Department of State, said US officials “are engaging with allies and partners at high levels over this”.

Meanwhile, US National Security Council spokesperson John Kirby admitted the administration is unsure if there will be more documents emerging over the coming days. At this point, he said on Monday, “we don’t know who’s behind this; we don’t know what the motive is”.

What do the documents appear to show, and which countries are involved?

The documents purport to reveal a wide range of information, including potentially sensitive information about the Ukraine war. They also indicate that the US has been eavesdropping on its allies, including South Korea and Israel.

Ukraine – If proven authentic, the leaks suggest that the US had been monitoring Ukrainian President Volodymyr Zelenskyy’s calls with defence and military officials by using signals intelligence.

They also reveal apparent weaknesses in the Ukrainian air defence systems and the size of military battalions.

Russia – The documents would also reveal that the US had penetrated the Russian military forces and the Wagner Group, a mercenary organisation, much more than previously understood.

The documents also reference details about the internal planning of the GRU, Russia’s military intelligence agency. Much of that information on Russian troop movements was gathered through human sources who could now be at risk.

United Arab Emirates – One document appears to show that US spies had caught Russian intelligence officers boasting that they had convinced the oil-rich UAE “to work together against US and UK intelligence agencies”.

The Emirati government on Monday dismissed the accusation that the UAE had deepened ties with Russian intelligence as “categorically false”.

Egypt – One leaked document appeared to show that Egypt planned to supply Russia with rockets and munitions. The paper, dated February 17, claims to summarise conversations between President Abdel Fattah el-Sisi and senior Egyptian military officials.

South Korea –  Another document appears to provide details of alleged internal discussions among top aides to South Korean President Yoon Suk-yeol that suggest that the US applied pressure on Seoul to help supply the US with artillery shells, which could then be sent to Ukraine.

It also indicates that the US spied on its ally, although South Korean officials claimed on Tuesday that details contained in the document are “untrue” and “altered”.

Israel – One document, which appears to be a CIA intelligence update from March 1, suggests Mossad, Israel’s intelligence agency, was encouraging its officers to participate in protests against their government’s plans to weaken the independence of the judiciary. Israel has denied Mossad played a role in the anti-government protests.