Belgium’s Brussels Airlines was hit with threats of a workers’ strike on Tuesday after staff said the airline violated an agreement with a labor union by forcing workers to staff flights to Israel, despite refusals from some employees.

Workers have cited the danger of flying into a war zone, and also the “ethical impact” of serving on the Israel-bound route.

According to The Brussels Times, a work agreement signed between the airline and the ACV Puls workers’ union specified that workers could volunteer to staff flights to Israel, but the airline could not compel them to do so if they refused.

“Staff have serious concerns about both their physical safety and the mental and ethical impact of these flights,” a union representative told the Belgian outlet.

“Nevertheless, Brussels Airlines refuses to give employees the option to refuse such assignments,” the union official said, adding that the situation was “particularly grave in the context of the ongoing war and genocide in Gaza.”