South Korean prosecutors have raided the offices of state sea traffic controllers, media reports said this morning, as part of a widening probe into a ferry disaster that left 300 dead or missing.

The move is the latest in an investigation with a broadening scope is broadening as officials try to assuage mounting public anger over the 16 April tragedy and the subsequent rescue efforts.

Earlier this morning, Prime Minister Chung Hong-Won stood down, the most high-profile resignation so far, after admitting he had not been able to prevent the accident or deal adequately with its aftermath.

Prosecutors are already holding all 15 surviving crew members who were responsible for sailing the vessel.

They face charges ranging from criminal negligence to abandoning passengers.

This morning, prosecutors raided the office of the state-run Vessel Traffic Services centre in the southern island of Jeju, Yonhap news agency and other media reported.

The VTS is the shipping equivalent of air traffic control.

Citing prosecutors working on the case, the reports added the VTS centre on the island of Jindo, the closest land to the wreck, was also being investigated.

The 6,825-tonne ferry was communicating with the two centres, primarily with Jindo, for about 30 minutes as it rapidly keeled over and sank, trapping around 300 people inside.

The bodies of 187 of the dead have been recovered, while teams of divers battling decompression sickness and powerful swells are still searching for a further 155.

Investigators have seized records of the VTS radio communication with the Sewol and surveillance video footage from both centres, Yonhap said.

A transcript of the communication between the ferry and the Jindo centre released earlier revealed panic and indecision among crew and sea traffic controllers in the crucial final moments, with neither able to make the call to evacuate passengers.

Businesses connected to ferry operator Chonghaejin Marine Company have also been raided over accusations of corruption, and travel bans are in place for eight current and former executives of the Korea Register of Shipping, the body responsible for issuing marine safety certificates.