One worker has died and three more were injured in an explosion on board a fast craft working for Azerbaijan's state oil company Socar in the country's portion of the Caspian Sea on Tuesday.

The vessel was en route to the shallow-water Gurgan Deniz offshore field, where seismic research activities are ongoing to determine the location of future wells.

The boat was delivering explosives for use in the seismic process, Socar said, adding that at this stage the cause of the explosion is unclear.

“Habil Mammadov was killed, another two Socar workers and (another person on board the vessel) were evacuated to shore and sent to hospital," the company added.

Two of the injured are in a serious condition, the hospital said.

The prosecutor's office has opened an investigation into the incident.