The US Department of Defense (DoD) is “working around the clock to figure out the scope and scale of the distribution of the information” and its veracity, spokesperson Chris Meagher told reporters on Monday. “It is highly classified, sensitive material that people in DoD and certainly other aspects of US government use to inform their work.”

He said the US has “engaged at high levels” with allies on the leak. At the White House, John Kirby, spokesperson for the National Security Council, said “we truly don’t know” whether more documents will be released.

While the Pentagon conducts a damage assessment, the Justice Department has opened a criminal investigation.

“We have been in communication with the Department of Defense related to this matter and have begun an investigation,” the department said in a statement. “We decline further comment.”

The secret documents appeared on social media sites in recent weeks. The materials reveal information on a wide range of topics, from US assessments of the war in Ukraine to intelligence gathered on diplomatic allies.

President Joe Biden was first briefed on the leak last week, “when we all got word that there were some documents out there, and he has stayed briefed — remained in contact with national security officials throughout the weekend”, spokesperson Kirby said at the White House.

Biden administration officials expect the leak to come up in the course of their regular contacts with allies, including in person when Biden meets in the UK later this week with Prime Minister Rishi Sunak. The UK and US are members of the “Five Eyes” group that shares some of their most sensitive intelligence, as are Canada, Australia and New Zealand.

One European official said the problem is mostly the messages conveyed by the leaked material — such as public confirmation that the US spies on allies, including Ukraine, and the implication that time may be on Russia’s side in its invasion because of Ukraine’s shortage of munitions and the limits of its air defences.

Mostly, the material confirms what allies already know, according to the official, who spoke on condition of anonymity.

Members of Congress are calling for briefings about the leak and how it happened. “The reports of intelligence leaks are incredibly concerning. The House Armed Services Committee is actively seeking answers from the Department of Defense,” Republican Representative Mike Rogers, chairman of the committee, said in an email.

(With assistance from Tony Capaccio and Jordan Fabian.)