WASHINGTON (NewsNation) — The United States is reassuring allies and partners it will get to the bottom of a major leak of classified U.S. documents that’s shaken Washington.

U.S. Secretary of State Antony Blinken reaffirmed to his Ukrainian counterpart the United States’s “enduring support for Ukraine … to defend its territorial integrity.”

These “deeply unfortunate” leaks, as the CIA director referred to them, reportedly reveal concerns raised by the U.S. that Ukraine will only be moderately successful in its crucial spring offensive against Russia.

Dmytro Kuleba, Ukraine’s minister of foreign affairs, tweeted Tuesday after their call that Blinken “vehemently rejected any attempts to cast doubt on Ukraine’s capacity to win on the battlefield.”

The U.S. government still has not individually authenticated the estimated 100 or so leaked files that ended up on social media, but said some date back to late February and early March.

The U.S. Department of Justice Department launched a criminal investigation. The Pentagon is also investigating internally.

“We take this very seriously and we will continue to investigate and turn over every rock until we find the source of this and the extent of it,” said U.S. Secretary of Defense Lloyd Austin.

“We have engaged with allies and partners at high levels over the past days, including to reassure them about our own commitment to safeguarding intelligence,” Blinken said.

The leaks reportedly reveal how American espionage is used to monitor U.S. allies and how countries interact with Russia and China. New reporting states that one document shows Serbia, the only country in Europe that has refused to sanction Russia, agreed to supply arms to Kyiv or has sent them already.

Serbia denied those claims Wednesday.

John Kirby, the spokesperson for the National Security Council, said it’s difficult to know how long it’ll take to track down who leaked these top-secret materials.