Nutella lovers might need to ration their supplies of the chocolate and hazelnut spread as French workers bring the world's biggest Nutella factory to a near-standstill in a showdown over salary negotiations.

Tensions have been mounting at the site in Villers-Ecalles in Normandy, where activists from the Workers' Force union have been barring trucks from entering or leaving the factory for a week.

The plant produces a whopping 600,000 jars of Nutella daily — accounting for a quarter of the world's production of a product cherished by children and adults alike. But hampered access to key ingredients could threaten the factory's productivity.

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Italian pastry maker Pietro Ferrero created Nutella in the 1940s. It now sells in 160 countries.

After six days of failed efforts to end the standoff, Nutella owner Ferrero on Monday started threatening to fine workers involved in the blockade, Ferrero France management said in a statement to CBS MoneyWatch: "Consequently, the plant management has duly informed the strikers illegally blocking the factory that it has obtained a court order foreseeing financial penalties for those continuing to strike past Monday June 3rd at 6.00 am."

But that threat didn't deter unions. Workers' Force says 160 of the factory's 350 workers are taking part in a walkout to demand 4.5% salary increases, one-time 900-euro bonuses and better working conditions.

"It's war, anger is mounting," union activist Fabien Lacabanne said in a statement.

He said the company agreed to a 1.7% raise for the lowest paid workers, and one-time bonuses between zero and 400 euros, which unions say isn't enough to offset rising living costs. Unions also complain of deteriorating factory conditions and mounting pressure to increase productivity.

Italian-owned Ferrero said it is trying to protect workers who aren't on strike, and wants to resume dialogue — but not until the workers stop blocking the factory. The next negotiation meeting is scheduled for June 13.

"The plant management is dedicated to protecting the large majority of the employees who are not striking and who wish to continue to carry out their usual activities. At the same time the plant management remains committed to guaranteeing fair and open dialogue," the company said, adding that it "hopes to resume its normal operations soon."

French workers regularly strike during salary negotiations and occasionally resort to more dramatic methods. The last strike to hit the Villers-Ecalles factory was in 2011.

The action comes amid anger among many low-income French workers at pro-business policies by President Emmanuel Macron seen as favoring the rich — and that prompted the yellow-vest protest movement.