BANGKOK, April 24 (Reuters) - A series of explosions ripped through a major army arsenal in central Thailand on Tuesday, prompting hundreds of families to flee, army commander-in-chief General Sonthi Boonyaratglin said.

There were no immediate reports of casualties at the armoury in the hills of Lopburi province, 150 km (90 miles) north of Bangkok, which stored mainly M-16 ammunition and artillery shells, Sonthi told Channel 7 television.

"We believe the heat might have caused the explosions because gunpowder can explode by itself when temperatures reach a certain level," Sonthi said on a day of 37 degree Centigrade (98.6 Fahrenheit) heat.

The blasts started around 8 p.m. (1300 GMT) and lasted for 90 minutes, prompting villagers and army families to evacuate to safe areas as a preventive measure, he said.

The armoury was only five years old and one of the army's biggest, army spokeswoman Colonel Sirichan Ngathong said .

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