The Swedish Armed Forces on Friday said one of its staff had accidentally left a USB memory stick with classified military documents on a public computer in Stockholm.

"We take this kind of carelessness very seriously," Colonel Bengt Sandstrom of the Military Intelligence and Security Service, or MUST, said in a statement.

According to Swedish daily Aftonbladet — which returned it to the armed forces after receiving it from an unidentified person — the memory stick was found in a computer in a public Stockholm library.

The military said it contained "both unclassified and classified information such as information regarding IED (improvised explosive devices) and mine threats in Afghanistan."

Two of the files were classified.

Countries other than Sweden, including the U.S., were also mentioned, the military said, but it did not mention which.

Aftonbladet said the documents included an analysis of the situation in Afghanistan as well as a classified U.S. intelligence report.

Sandstrom was scheduled to meet later Friday with defense attaches from the countries mentioned in the documents to discuss the incident.

"It's primarily a matter of security for our soldiers," he said, adding that carelessness with classified material is an offense that could lead up to six months in prison.

The armed forces said the employee had confessed to the misplacement and that a preliminary technical investigation confirmed it belonged to him.

A civil investigation would be launched, it said, and the contents of the memory stick would be analyzed to determine the damage its misplacement could have caused.