Soldiers at base next to Cologne airport warned not to drink tap water.

German authorities have sealed off a military base in Cologne as they investigate the potential contamination of the water supply.

A German defence ministry spokesman confirmed on Wednesday that an investigation of suspected sabotage was under way at the Cologne-Wahn barracks. A warning has been issued to those at the base not to drink the tap water and to remain on site.

The base, which sits next to Cologne airport, is home to the German Air Force office responsible for aircraft used for travel by Chancellor Olaf Scholz and ministers.

Following indications of a disturbance, a hole was discovered in a fence, it was reported. Samples have been taken due to suspicion that the water supply could have been contaminated.

Thousands of people at the base, which employs 4,300 soldiers and 1,200 civilians, have been ordered not to enter or leave the barracks and advised not to drink tap water while the police, military police and the military intelligence agency carry out their probe.

A spokesperson for the territorial command in Berlin confirmed to news agency Reuters that the base in Cologne-Wahn had been sealed off, saying the army took the case “seriously”.

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

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

In July, 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.