At least 15 labourers were injured, seven of them seriously, when a fire broke out in an oil mill following an explosion in an oil tank on Hisar-Tohan road near Hisar on Sunday, the police said. The explosion in the oil tank took place following a leakage in the boiler, they said. It took five hours for fire brigade personnel from Hisar, Tohana, Barwala and Narwana to extinguish the fire. The impact was so powerful that the roof and walls of the mill crumbled and it soon turned into a heap of debris. All the injured were shifted to Hisar and were admitted to different private hospitals. Seven persons -- Rahul, Pankaj Ram, Suresh, Bhim Raj, Ganesh, Ramesh and Narender received burn injuries ranging from 90 percent to 100 percent, police said. Their condition is stated to be critical, said police. Eight labourers received 20 percent to 85 percent burn injuries and have been admitted to private hospitals in Hisar. The police chief of Hisar district, Manisha Chaudhary visited the spot and hospitals to take stock of the situation. The incident comes close on the heels of a boiler blast in the NTPC plant in UP which left 33 persons dead.