All rail traffic in northern Germany was brought to a halt after cables vital for the rail network were intentionally cut in two places on Saturday morning with authorities working to identify those responsible for the sabotage.

The federal police are investigating the incident, German Interior Minister Nancy Faeser said, adding the motive for it was unclear.

The disruption raised alarm bells after NATO and the European Union stressed the need to protect critical infrastructure following what they called acts of sabotage on the Nord Stream gas pipelines last month.

"It is clear that this was a targeted and malicious action," German Transport Minister Volker Wissing told a news conference.

"Cables that are essential for train operations were deliberately and wilfully cut," Wissing added.

Police located two crime scenes, one of which is in the Berlin district of Hohenschönhausen, a police spokesperson said, without specifying the location of the other, in North Rhine Westphalia.

A security source said there were a variety of possible causes, ranging from cable theft – which is frequent – to a targeted attack.

Omid Nouripour, leader of the Greens party, which is part of Chancellor Olaf Scholz's federal coalition, said anyone who attacked the country's critical infrastructure would receive a "decisive response."

"We will not be intimidated," he wrote on Twitter.

Chaos before election day

"Due to sabotage on cables that are indispensable for rail traffic, Deutsche Bahn had to stop rail traffic in the north this morning for nearly three hours," the state rail operator said in a statement.

Deutsche Bahn (DB) had earlier blamed the network disruption on a technical problem with radio communications. Spiegel magazine said the communications system was down at around 6:40 a.m. (4:40 a.m. GMT). At 11:06 a.m, DB tweeted that traffic had been restored, but warned of continued train cancellations and delays.

The disruption affected rail services through the states of Lower Saxony and Schleswig-Holstein as well as the city-states of Bremen and Hamburg, with a knock-on effect on international rail journeys to Denmark and the Netherlands.

They came the day before a state election in Lower Saxony where Scholz's Social Democrats are on track to retain power and the Greens are seen doubling their share of the vote, according to polls.

Lines rapidly built up at mainline stations including Berlin and Hanover as departure boards showed many services being delayed or canceled.