Vandals armed with hammers have caused serious damage to a fourth-century mosaic inside an ancient synagogue in the town of Tiberias on the Sea of Galilee, police and media reports said.

The attack on the Hamat Tiberias synagogue took place late on Monday with the vandals spray painting slogans on the walls and gouging several holes in the middle of the intricate mosaic on the floor, press reports said.

"Unknown attackers painted slogans on the exterior and interior walls and valuable mosaics were seriously damaged with a hammer," police spokesman Micky Rosenfeld told AFP.

Press reports said the main suspects were believed to be ultra-Orthodox Jews protesting against archaeological work which they claim damages Jewish graves.

One of the Hebrew slogans sprayed on the walls read: "A site for every grave."

Last month, another rare mosaic from the Byzantine era was smashed up and covered with Hebrew graffiti at the Khirbet Hanut site in the Elah Valley just outside Jerusalem.

On the walls was written: "The magnitude of destruction equals the magnitude of desecration."