The International Labour Organisation says the Fiji interim government must undertake independent investigations without delay into allegations of assault, harassment and intimidation of trade union leaders and members.
It has also called on the regime to allow ILO investigators back into the country after a mission in September was abruptly cut short.
In a new report, the ILO's Committee on Freedom of Association is critical of five countries where workers' rights are being threatened.
The others are Argentina, Cambodia, Peru and Ethiopia.
The Committee says the Fiji regime should quickly discuss the return of its Direct Contacts Mission.
The mission had intended looking into complaints made by trade unins about the lack of freedom of association.