The Geilenkirchen air base, near the border with the Netherlands, is where the alliance's Airborne Warning and Control System (AWACS) aircraft are based.
A post on the NATO AWACS fleet's social media account announced that "we raised the security level" but did not elaborate on the potential threat.
🚨We raised the security level at NATO Airbase Geilenkirchen based on intelligence information indicating potential threat. All non-mission essential staff have been sent home as a precautionary measure. The safety of our staff is our top priority. Operations continue as planned.— NATO AWACS (@NATOAWACS) August 22, 2024
It said operations "continue as planned".
German news agency DPA said a reporter saw police cars in the grounds of the air base.
Police confirmed a deployment on Thursday night but gave no details, and offered no information on Friday morning.
Last week, a major German air force base near Cologne was locked down for several hours amid fears that its water supply might have been tampered with.
An investigation found no evidence of sabotage.
There were also reports of suspicious observations at Geilenkirchen and a person was briefly detained for questioning near the base, DPA reported at the time, but there turned out to be nothing untoward.
The incidents come at a time of jitters about the possible vulnerability of infrastructure to Russian sabotage.
On Thursday, prosecutors in Flensburg, in Germany's far north, said they were investigating suspicions of espionage for the purpose of sabotage, without elaborating on who might be behind it.
German media reported that drones had been spotted over a chemical park in Brunsbuettel, on the North Sea coast.
