According to French paper Le Parisien, holidaymakers and residents in the region recently called the police after they saw Swiss military helicopters airlifting water from the Lac des Rousses, which lies just over the French border in Franche-Comté.
The action was part of a Swiss government-funded operation launched last week to deliver water from lakes across Switzerland to cows grazing in the Swiss Jura and other mountain pastures, where the recent heatwave has led to water shortages.
But authorities in the French Jura, where water restrictions are in place after similarly blistering temperatures, said their Swiss neighbours did not have permission to help themselves to the Lac des Rousses, reported French radio station France Info on Sunday.
“We were astonished. We approached the Vaud authorities who immediately suspended their operation,” Renaud Nury, secretary general of the Jura prefecture, told France Info.
Nury also pointed out that the act of flying over French territory without permission could be considered "illegal".
Speaking to Swiss daily 20 Minutes the mayor of Les Rousses, Bernard Mamet, said: “It all happened without our knowledge, even though the commune owns the lake. That’s not normal.”
For their part, the Swiss army claimed to have told Paris of their intention but the message was not passed on to the Jura authorities.
“On Wednesday we sent an official request to the French state, which gave us the green light,” Daniel Reist, spokesman for the Swiss army, told 20 Minutes.
According to him, the army chose the Lac des Rousses for its proximity to many of the cows in need.
“We saved 15 minutes per flight,” he said.
Authorities on both sides of the border are expected to meet today to resolve the matter.
In addition to the Swiss Jura, cows in the Fribourg alps and the canton of St Gallen are also being helped by the Swiss army.
Some 20,000 Swiss cows are thought to be in need of drinking water. Each cow can slurp down around 100-150 litres per day.