Switzerland is planning legal action against Nigeria over the seizure and detention of a Swiss-flagged oil tanker in January 2018.

Reports by various online maritime websites said San Padre Pio and four of its original 16 crewmembers, all of whom are Ukrainian nationals, had since remained under Nigerian government custody in the Bay of Biafra for alleged diesel smuggling and illegal entry into Nigerian territorial waters.

The 12 other crewmembers were released a month and a half following their seizure by Nigerian authorities. However, the four crewmembers who remained in the country had been reportedly enduring “difficult” living conditions, the Swiss Foreign Ministry said in a statement.

The decision to file the case with the International Tribunal for the Law of the Sea came after diplomatic efforts by the Swiss government failed to ensure the release of the vessel and its remaining crew.

The Foreign Ministry decried the seizure and subsequent detention of San Padre Pio and its crew as “a violation of international law” and stated that Switzerland should exhaust all options “to safeguard its interests.”