Coastguard officials described it as a minor explosion and other officials said operations at the port were not affected.
"There is no disturbance, there is nothing to worry about," Amelia Velasquez, the supervisor of the Bauan terminal at Batangas, told Reuters. The 1,115 deadweight tonne vessel was carrying about 1,000 litres of ethyl alcohol, said harbour official Romy Alfofera.
The explosion occurred when workers tried to open an auxiliary valve of the engine, he said. Three workers received minor injuries and were taken to hospital. The ship and its cargo were not affected.