Tonight’s sightings come after drones were seen flying over a Belgian military base, Kleine-Brogel, for three nights in a row until Monday, according to the country’s defence minister.

Theo Francken said an investigation was under way, calling it “a clear mission targeting Kleine Broge”.

The drones reportedly flew off towards the Netherlands after a helicopter was deployed near the base on Monday, reports national broadcaster VRT.

Francken told Belgian radio on Monday that it appeared to be espionage, but avoided speculation on who was behind it.

“I have some ideas, but I’m going to be cautious,” he said.

Drones were also spotted flying above other military air bases – the Leopoldsburg, in central Limburg province, and Marche-en-Famenne in the south-east of the country.