A Lithuanian-flagged ferry carrying 335 people is powerless in the Baltic Sea but no one is reported injured, according to a spokesman for Danish ferry company DFDS.
"A rescue operation is ongoing," the head of the Kaliningrad Maritime Rescue Service told Interfax news agency.
The vessel, named Regina Seaways, is owned by the Danish shipping company DFDS according to Denmark's public broadcaster NRK, citing Lithuanian media.
Earlier reports from the Lithuanian military suggested the vessel had caught fire but the DFDS spokesman contradicted this saying: "There was vibration and there was smoke, but no fire was noticed."
He could not say whether the passengers and crew were in any danger and said the ferry would have to be towed to Klaipeda port in Lithuania.
A Lithuanian air force spokesman said a military helicopter was sent to the ferry when it sent out a distress call at 12.45 CEST.
The ferry was heading to Klaipeda, a port town, from Kiel, Germany when it experienced technical problems.
Lithuania's Defence Ministry said "the fire has been extinguished" and that four Lithuanian military ships were heading towards the ferry and would reach it by 17.00 CEST.