The defence ministry said it was also taking emergency measures to prevent the flames engulfing a nearby military airbase which has MiG-29 fighter jets and munitions on site.
Security Council Secretary Oleksandr Turchynov and Defence Minister Stepan Poltorak rushed to the scene to oversee the rescue operations.
"We are facing a serious threat... Because 50 metres (yards) from the edge of the fire, we have a military installation stocked with an arsenal," Turchynov said in televised comments.
Fourteen people -- including seven rescue workers -- were being treated for burns and other injuries.
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The lightning speed with which the flames spread to neighbouring petrol stations and fuel tanks resulted in initial confusion among emergency workers and contradictory toll reports.
Interior Minister Arsen Avakov said officials were still trying to establish contact with three of the 30 firemen who were initially sent to the spot of the first blast.
But more explosions soon followed and dozens of additional firetrucks and ambulances were dispatched to the scene, where thick black smoke billowed into the sky.
Videos posted on the Internet showed the force of the first explosion and the heatwave it produced setting ablaze several firetrucks and ambulances that had been rushed to the scene.
A fireman wearing a silver heat-resistant suit and mask was seen running away from the disaster for his own safety.
"Everyone -- leave. Whoever is still here -- leave," one fireman said in a video.
Ukraine's emergencies ministry began evacuating residents living within two kilometres of the fuel depots.
"If necessary, we will evacuate people living within 10 kilometres (of the blast site)," the ministry statement added.
Security Council chief Turchynov said the accident most likely resulted from human error and "safety rule violations".