New Zealand officials are urging restraint as a Sea Shepherd boat pursues a fishing boat the group says is poaching toothfish in Antarctic waters.
The environmental group says it has the Nigerian-flagged Thunder under citizen's arrest, saying it has been flagged by Interpol as a ship that is operating illegally.
The Ministry of Foreign Affairs and Trade (MFAT) has confirmed Interpol has issued a notice seeking information on any criminal activity by the crew of the Thunder.
The ministry said the boat was listed for illegal fishing by the Commission for the Conservation of Antarctic Marine Living Resources.
It said it urged all involved to show restraint in the remote and hostile Southern Ocean.
The captain of the Sea Shepherd ship Bob Barker, Peter Hammarstedte, said if it had to, it would use its superior bulk to stop the Thunder from fishing.
He told Morning Report that Sea Shepherd would keep the pressure on the other boat, saying that since his crew found it two days ago, it had suspended its fishing operation.
Mr Hammarstedt said Sea Shepherd was waiting for the New Zealand or Australian authorities to take over the arrest.
Sea Shepherd has been conducting a campaign called Operation Icefish designed to catch toothfish poachers in Antarctica.