At least 100 people were feared dead on board the Iranian frigate IRIS Dena after the vessel sustained a possible submarine attack off the Sri Lankan coast on Wednesday morning, according to Sri Lankan navy sources.

Local media earlier reported that around 180 people were on board the frigate. Sri Lanka Navy launched rescue operations immediately after receiving a distress call reporting that the vessel was sinking. A total of 35 people from the frigate were rescued and transferred to the Karapitiya Teaching Hospital in the southern city of Galle.

The incident occurred outside Sri Lanka's territorial waters, according to the reports citing Foreign Minister Vijitha Herath.