The usual hum of traffic on the A1 highway was replaced by the din of F/A-18s touching down on the carriageway.

A large stretch of the A1 was shut down for 36 hours for the test -- a relatively common exercise for NATO members but one rarely seen these days in neutral Switzerland.

Departing from the Payerne military airfield, located near the closed motorway section, the first F/A-18 fighter jet landed without a hitch at around 10:00 am (0800 GMT), as planned, the military said.

The three other F/A-18s also landed successfully on the highway, which crosses Switzerland from the French border in the west to the Austrian border in the east.

The so-called Alpha Uno exercise is aimed at making sure the Swiss airforce can keep operating if its bases are hit in the event of a conflict.

Measuring just 25 metres (82 feet) across, the four-lane motorway is also narrower than the usual landing strip, and can only be used once the central safety rail has been removed.

After the initial landing on Wednesday, the track conditions were checked.

Even small rocks and debris can cause considerable damage to jet engine turbine blades.

Another landing and take-off cycle was due to take place later on Wednesday.

The Swiss airforce operates around 30 rapidly ageing F/A-18s.

To replace them, the Swiss government has chosen a fleet of 36 F-35 stealth fighters, at a cost of more than $6 billion.

Deliveries from US manufacturer Lockheed-Martin are due to begin in 2027 and last until 2030.

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

A key facet of its national identity, Swiss neutrality traces its roots back to 1516 and has been internationally recognised since 1815.