A German military base next to Cologne airport was temporarily sealed off and soldiers there were told not to drink tap water as authorities investigated possible sabotage, a security source said Wednesday.

The water on the base, which has 4,300 soldiers and 1,200 civilian employees, might have been contaminated after someone forced their way in, creating a hole in the fence, the source added, confirming a report by Spiegel magazine.

The barracks were reopened in the afternoon, but the tap water ban was maintained as a precautionary measure pending the result of tests, the source said.

The base in Cologne-Wahn is home to the fleet of military aircraft used for travel by Chancellor Olaf Scholz and his ministers.

A NATO base in nearby Geilenkirchen, housing the alliances' fleet of AWACS surveillance planes, raised its security level after the incident in Cologne but did not shut down access.

"The base (in Geilenkirchen) was at no time sealed off. We checked the water, it is okay, and the base is accessible," the spokesperson said.

A German military spokesperson had previously said that the Geilenkirchen base had been sealed off.

NATO has warned in the past of a campaign of hostile activities staged by Moscow, including acts of sabotage and cyberattacks, but there was no indication of who might have gained illegal access to the Cologne base.

In June, NATO Secretary-General Jens Stoltenberg said the Western military alliance saw a pattern evolving and that recent attacks were a result of Russian intelligence becoming more active.

Several countries such as Poland, Germany, Britain, and the Czech Republic have reported incidents in the past months.