Unidentified drones were spotted over a Belgium military base housing U.S. nuclear weapons, sparking security concerns and an investigation into a potential spying operation.

Liège Airport has also reported drones and flight diversions. "All Belgian airspace is closed," announced Liège Airport spokesperson Christian Delcourt. Liège Airport has reportedly activated its internal crisis centre.

BRUSSELS (Reuters) -Brussels Airport is currently closed after the reported sighting of a drone, said the Belgian air traffic control service and a spokeswoman for the airport. "There are no flights currently landing or taking off," said the spokeswoman, adding she could not estimate how long the airport would remain closed.

Air traffic at Brussels Airport was suspended on Tuesday evening as a safety precaution following a drone sighting, according to local media. Just before eight o’clock (1900GMT), there was a report of a drone over the airport grounds," air traffic controller Skeyes spokesperson Kurt Verwilligen told daily Het Laatste Nieuws (HLN).

The drones are trying “to see where the F-16s are, where the ammunition is, and other highly strategic information,” Belgian Defense Minister Théo Francken told RTBF.

Belgium's defense minister said that more counter-uncrewed aerial systems "are urgently needed" after three drones spied on the Kleine-Brogel base.

They [the drones] come to spy — to find out where the F-16 fighters are, where the munitions are stored, and other strategically important information,” Francken said. Read also: Unknown drones spotted over key Belgian military base;