Hundreds of Sri Lankans have been left homeless after explosions at an ammunition depot on the edge of Colombo flattened houses and killed a soldier, an official said Tuesday.

Thousands of residents are yet to return to their homes after a huge fire on Sunday night at the military store triggered the blasts and forced them to flee.

The area's top official, D.S. Bandara, said surveys of the affected zone estimate 300 homes have been destroyed and another 600 damaged after shrapnel and unexploded bombs fell on villages.

"This is the initial estimate we have, but the numbers could rise as people go back to their properties and assess the damage," Bandara told reporters.

"Our figures show 18,620 people were living within this area and they are (currently) now homeless," Bandara said, adding they will be allowed to return to their villages in coming days to check their homes after the military declares the area safe.

Residents were in the meantime sheltering at schools and temples in the area, with disaster officials providing food.

The military is not allowing journalists inside the sprawling Salawa army complex, 36 kilometres (22.5 miles) east of Colombo, but aerial photos show buildings destroyed and only a water tower intact.

At least one soldier burnt to death while about 50 people were treated for minor injuries in the explosions and fire that was only extinguished on Monday.

Prime Minister Ranil Wickremesinghe pledged to rebuild the destroyed homes after visiting the area, his office said.

Police have launched a criminal investigation into the cause of the fire, which has caused the worst destruction since the end of Sri Lanka's decades-long separatist war in 2009.

It was the second time in less than a month that residents had been forced to leave their homes after floods hit the capital in May when the rain-swollen Kelani river burst its banks.