An investigation has been launched after vital rail network cables were allegedly sabotaged in northern Germany, bringing rail traffic to a near three-hour halt.

Authorities said attackers had cut the cables in two places, without identifying who might be responsible.

Germany's Transport Minister Volker Wissing told a news conference the federal police were investigating.

"It is clear that this was a targeted and malicious action," he said.

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

"We can't say much at the moment, it is too early," said a security source who declined to be named.

The source said an intensive investigation into the incident had begun and a there were a variety of possible reasons for it, ranging from simple cable theft — which was frequent at the moment — to a targeted attack.

Neither the federal police nor the interior ministry immediately replied to requests for comment.

State rail operator Deutsche Bahn (DB) said in a statement: "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."

DB had earlier given the cause of the network disruption as a technical problem with radio communications.

Rail traffic was still patchy on Saturday afternoon after being restored, it said, warning of train cancellations and delays.

The disruptions affected rail services through the states of Lower Saxony and Schlewsig-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.

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

Station staff were seen attempting to give advice to passengers as the delays lengthened.

Reuters