The cartridge, labelled "Secret, US Government Property", was found in a metal case closed with a digital lock, and was wrapped in a mailbag of the US defence department, according to news reports.
"US government property marked secret has been found," an embassy official confirmed to AFP, declining to give further details.
The cartridge was found by a worker in a scrapyard that collects metal from around the Taszar airbase in southern Hungary, used by US troops. He took it to a Hungarian TV station, which passed it on to the US embassy. The station broadcast a report on the find, showing the Walkman-sized silver and black cartridge, on which could also be read the inscription "This medium is classified. Protected from unauthorized disclosure in compliance with applicable executive orders, statutes and regulations." The bag also contained two keys and the password necessary for decoding the message on the Colorado DT-1000 type data cartridge, according to the TV2 private television station.
But experts, who estimated that the cartridge had been discarded 12-18 [?months] ago, said it was unlikely non-experts could retrieve the information stored in the cartridge.
"The person who stored the keys and the password alongside the cartridge failed to respect the most basic rules of secrecy," expert Janos Kis told AFP.
The keys, which must be inserted into a special decoder, and the secret password which must be typed in to decode the information on the cartridge, should not be stored together, he explained. "But it is highly unlikely that any device used by civilians in Hungary could decode or read the data cartridge," Kis said.
The Taszar base has operated since 1995 as a supply base for US troops in the International Forces (IFOR), then Stabilisation Forces (SFOR) peacekeeping forces in Bosnia. From May this year, US marines used it for bombing mission in neighbouring Yugoslavia.
The TV station involved blacked out the faces of all US officials involved, in its report on the find.