An almost 7,000-year old stone tablet found in Bulgaria bears carvings that might turn out to be one of the world's oldest inscriptions, a prominent Bulgarian archaeologist said Thursday.

"These signs are unique and apparently bear a meaning," Nikolai Ovcharov told a press conference.

Ovcharov said he had received the tablet from a private collector who had unearthed it 20 years ago.

The collector asked to remain anonymous, because he risked criminal prosecution for looting or criminal possession of antiquities.

The tablet, about three inches, carries five distinct signs each made up of two elements, Ovcharov said.

"This could be the prototype of a script," he added.

Two similar tablets also dating back to the 5th millenium B.C. have also been found in Bulgaria many years ago.

It could be argued that their carvings, although rather schematic, are part of the same proto-script, Ovcharov said.