Officials said the ship, Sagar Bhushan, was under repair at the shipyard when an explosion rocked a section of the vessel around 9.15am following an apparent gas leak near an air-conditioned compartment.

The deceased have been identified as C.S. Unnikrishnan, K.B. Jayan, Gibin Reji, M.M. Ramshad and M.V. Kannan, all Kerala residents from nearby areas.

The condition of a worker identified as Sreeroop is said to be critical. He has suffered extensive burns. The others in hospital have also suffered burns.

Cochin Shipyard has announced a compensation of Rs 10 lakh each to the next of kin of those killed and decided to take care of the medical expenses of those injured.

Tuesday being a holiday at the shipyard because of Mahashivaratri, those working at the dry dock were contract labourers. Around 20 people were working on the ship.

Officials said the blast took place when the workers were cutting and welding near the tank meant to carry potable water.

A probe has been ordered to find out what caused the blast, but officials have ruled out a sabotage.

Cochin Shipyard chairman and managing director Madhu S. Nair told reporters that the blast was probably cause by a gas leak. "The loud blast indicates the presence of some kind of gas entrapment in the tank the men were working on," he said. "As we follow the best of safety protocols, an internal investigation will look into any violation of norms," Nair added.

The drillship had arrived for extensive repairs four months ago. Dry-docked on January 12, it was scheduled to be undocked by the end of this month.

Shipyard officials said the blast had not caused any major damage to the ship as the explosion had occurred in the confined area of the water tank.