Switzerland has officially signed a contract to buy three dozen F-35 fighter jets from the U.S. as part of a planned modernization of the Swiss Air Force, AP reports.

Switzerland's head of arms and the director of the F-35A program have signed an agreement to buy the planes, which will be delivered between 2027 and 2030, worth just over 6 billion Swiss francs.

The planned purchase has already been signed by the United States, where the manufacturer Lockheed Martin is headquartered, the Swiss government said in a statement. The purchase price will include mission-specific equipment, weapons and ammunition, as well as logistics, mission planning and training support.

Opponents managed to petition to put the planned purchase to a referendum late last month, but Swiss government officials say voters have already agreed to it and the contract should already be signed.

Opponents say the planes are too expensive, would tie the Swiss security service too closely to the United States, and are not suitable for Switzerland's needs. The Swiss Air Force mainly uses such fighters for air patrols in European skies, not in conflicts abroad.

The country's constitution states that Switzerland is a neutral country, which limits its ability to engage in foreign conflicts.