The scare follows a trespassing attempt at the same site last week, which came around the same time as a separate case of suspected sabotage at a German military base.

"We raised the security level at NATO Airbase Geilenkirchen based on intelligence information indicating potential threat," the base said in a post on social media platform X late Thursday.

Germany -- a key ally of Kyiv -- has been on high alert for sabotage and attacks on military facilities.

A spokesman for police in Cologne, close to the NATO base, told AFP on Friday that there was an "ongoing police deployment" at the site but said "the police cannot give further details due to investigations which are currently ongoing".

"We are in constant contact with the relevant security and judicial authorities as well as those at the NATO base in Geilenkirchen," he said, adding that officers from the Cologne force "have been deployed to support security measures coordinated by the international military police around the NATO air base".

The Geilenkirchen base close to the Dutch border houses AWACS reconnaissance aircraft and has been in use by NATO since 1980.

In last week's suspected trespassing incident, NATO said an individual attempted to enter the base but was stopped and sent away.

In the second case, the Bundeswehr base in Cologne-Wahn was locked down after a hole was discovered in a fence near drinking water storage facilities.

But test results eventually showed that the tap water was not contaminated, according to the German army.

No link between those two incidents has been established.

In April, investigators arrested two German-Russian men on suspicion of spying for Russia and planning attacks in Germany -- including on US army facilities -- to undermine military support for Ukraine.

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