The world's gem capital, Antwerp, has called for extra security after a local Arab group claimed it could by attacked by militant groups like Hamas.
Antwerp handles 80% of the world's uncut gems
The Arab European League (AEL) said the Belgian port could become a target if its 20,000-strong Jewish community did not denounce Israeli policies.
"The diamond sector openly supports the Zionist regime," Ahmed Azzuz, the group's local leader, added.
Local police said rigorous security measures were already in place.
"They're already doing everything they can," a spokeswoman told Reuters.
In an interview with a local magazine, Mr Azzuz said Hamas planned to attack foreign targets following the assassination of its spiritual leader Sheik Yassin and that Antwerp was an "obvious target".
"Every year, 200 Belgian-Israeli reservists leave for Israel to kill innocent civilians," he was quoted as saying.
The Diamond High Council, which represents gemstone traders, has filed a complaint against the AEL.
It accuses the group of "intimidation" and "threatening behaviour".
"It is the first time the diamond sector has been named as a target in such an explicit manner," spokesman Peter Meeus told Reuters.
Antwerp handles about 80% of the world's uncut gems.