The northern port is home to German naval installations and shipyards of the defence giant Thyssenkrupp, which builds submarines there.

A Chinese man was detained by security guards before he was arrested by police after taking photographs at the Kiel-Wik naval base on December 9, according to local media.

"I can confirm that the State Criminal Police Office of Schleswig-Holstein state is conducting an investigation against a Chinese citizen on suspicion of espionage," a police spokeswoman told AFP.

Prosecutors in the state confirmed to AFP that they had launched "an investigation against a Chinese citizen on suspicion of taking pictures" in a case considered "relevant to state security".

They also said the suspect had in the meantime been released from detention.

Public broadcaster WDR said police were evaluating images on his phone and the Federal Office for Military Counterintelligence was also involved in the case.

Tensions have been building in the Baltic Sea since the start of the war in Ukraine, with European countries denouncing "hybrid attacks" blamed on Russia.

Sweden has been investigating a Chinese ship since November in the case of the sabotage of two submarine telecommunications cables.

Germany in early October said it had arrested a Chinese woman accused of spying on the country's defence industry while working in a logistics company, including at Leipzig airport in eastern Germany.

Named only as Yaqi X., she allegedly reported to another suspected Beijing agent now under arrest, Jian G., who was working in the office of a German far-right member of the European Parliament, Maximilian Krah.

News magazine Der Spiegel, citing unnamed security sources, said that 38-year-old Yaqi X. had especially targeted the arms giant Rheinmetall, which is involved in making Leopard tanks and uses Leipzig airport for cargo flights.

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THYSSENKRUPP

RHEINMETALL