Officials in the western town of Mechernich said a hole was discovered Thursday afternoon in a fence on the grounds of a water storage site that supplies Mechernich, several surrounding towns and a nearby military base.

The incident sparked concern the water may have been tampered with, just two days after a similar scare at the Cologne-Wahn military base in the same state of North Rhine-Westphalia.

Initial analysis of water samples however showed that contamination with biological or chemical substances "can be ruled out", Mechernich authorities said on the town's website.

"This has significantly eased the situation," the statement said.

Further tests were still being carried out, the results of which are expected after the weekend.

An earlier ban on consuming tap water was lifted for some 10,000 residents in and around Mechernich, but they were nevertheless advised to boil the water before using it for drinking, cooking or teeth brushing.

A spokesman for the Bundeswehr military confirmed the incident when contacted by AFP.

The cut fence at the water site in Mechernich, located near a Bundeswehr base, was spotted after patrols were increased this week following possible sabotage at the Cologne-Wahn base.

The base near Cologne-Bonn airport was temporarily sealed off Wednesday after a hole was discovered in a fence near drinking water storage facilities.

Tests later showed "unusual values" in the water, the military said.

Around the same time, NATO announced there had been an attempted trespassing incident late Tuesday at its base in the western German town of Geilenkirchen, which houses AWACS reconnaissance aircraft.

No link was established with the Cologne incident, and NATO said there were no concerns about the water supply at Geilenkirchen.

Defence Minister Boris Pistorius on Wednesday said the security scares "show we must remain vigilant".

Germany -- a key ally of Ukraine -- has been on high alert for sabotage and attacks on critical infrastructure in the country in the wake of Russia's 2022 invasion of Ukraine.

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.