The US Defense Security Cooperation Agency (DSCA) wants Switzerland to assume additional costs, but Bern says it is sticking to the agreed price of just over six billion Swiss francs ($7.5 billion at current exchange rates).

"In recent months, challenges have arisen," Switzerland's Federal Council government said in a statement.

"From the DSCA's point of view, Switzerland will have to bear additional costs due to the high inflation in the US in recent years and the sharp rise in raw material and energy prices in the aftermath of the Covid pandemic."

The agency told Switzerland in February that in its view, "the fixed price was a misunderstanding, without quantifying the additional costs".

"The Federal Council is sticking to the validity of the fixed price," it said, adding that the amount had been contractually agreed and had been publicly confirmed by the US embassy in Bern.

National armaments director Urs Loher said that agreeing to DCSA's new terms "results in potential additional costs of between $650 million and $1.3 billion."

The wide gap between the two figures was due to the potential impact of US tariffs, possible inflation trends and geopolitical uncertainties, he said.

- Deliveries due in 2027 -

The DCSA is an agency of the US defence ministry which works to build the capacity of foreign security forces.

Switzerland buys the planes through the DSCA's Foreign Military Sales programme, which then settles the purchase with the manufacturer through its own contract.

Bern said "a diplomatic solution must be sought" to the disagreement and has charged the Swiss defence ministry with coming up with a proposal to resolve the dispute, the statement said.

The government won a referendum in September 2020 by a razor-thin margin, which approved the military spending six billion Swiss francs on a new fleet.

The F-35A combat aircraft -- already used by the US Air Force and several European countries -- was chosen in June 2021 instead of the Airbus Eurofighter, the F/A-18 Super Hornet by Boeing, and French firm Dassault's Rafale.

Two Swiss parliamentary committees launched an investigation into why the model won despite a series of technical problems reported in the United States. They also questioned their high cost.

The contract was signed in September 2022, when six billion Swiss francs was the equivalent of $6.2 billion.

Terminating the contract would have "significant consequences", the government said Wednesday.

"Switzerland would no longer be able to guarantee the safety of the airspace and the population from 2032, as the previous F/A-18 fighter jets are reaching the end of their service," it said.

The first deliveries are due in 2027.

Switzerland's long-standing position has been one of well-armed military neutrality and the landlocked country has mandatory conscription for men.