Boris Horvat, AFP | Swiss army helicopters have been flying water to cows on the country’s high Alpine pastures in the wake of a record heatwave

Swiss officials have issued a formal apology after it emerged army helicopters had crossed the border with France on Thursday in an unexpected incursion to draw water for thirsty Swiss cows.

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The aerial operation to scoop up water caught authorities responsible for Rousses Lake in the Jura mountains by surprise last Thursday. The helicopters also startled swimmers and fishermen enjoying the beaches of the lake in eastern France.

Christophe Mathez, deputy mayor of the Les Rousses commune, said officials had "no idea this operation would occur" - and that the Swiss neither requested authorisation nor warned before descending.

On Monday, local officials in the Vaud canton of Switzerland sent a letter to their counterparts across the border apologising for the incident, which they described as a “misinterpretation” of a bilateral agreement that allows the Swiss air force to fly over France – but not to take its water.

French officials confirmed they received the letter of apology, along with an offer of reimbursement.

The Swiss army has been pumping water for livestock from its own Neuchatel and Joux lakes since last week, according to a statement from the Swiss Department of Defence. The federal government is bankrolling the operation, expected to continue no longer than August 4, with military aid disaster relief funds.

Thirsty cows produce less milk, of particular concern in the mountainous Jura region of France and Switzerland. Dairy farmers there provide milk to producers who make prized cheeses including the French Comté and Morbier varieties as well as the Swiss Tête de Moine.

(FRANCE 24 with AP)

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