Sri Lankan flag
An air force officer holds Sri Lanka's national flag as the sun sets at Galle Face Green in Colombo, Feb. 3, 2016. REUTERS/Dinuka Liyanawatte

One soldier was killed and thousands of people were evacuated after a fire and a series of explosions on Sunday at one of Sri Lanka's biggest ammunition dumps, where the army stores heavy weapons, officials said.

Explosions at the army camp at Salawa, 20 miles east of the capital, Colombo, continued for more than five hours and were heard more than 7 miles away, local residents said.

The cause was not immediately known and the government ordered an investigation by the police criminal investigation department.

Local television stations showed rocket propelled grenade shells and shrapnel that had landed in the middle of a road.

"The fire started in a small arms store and spread. There were heavy weapons such as artillery shells stored in the proximity but the damage is still unknown," Sagala Ratnayake, minister of law and order, told media.

One soldier was killed and one injured in the blasts, an army spokesman said.

Sources from two nearby hospitals said at least five people were being treated for injuries. The soldier died on admission due to loss of blood, they said.

Pradeep Koddipily, spokesman for Sri Lanka's Disaster Management Center, said residents living in a 3-mile radius had been asked to evacuate immediately. Thousands of people have already been evacuated fearing explosions.

Rohitha Fernando, chief operating officer at Colombo Fire Brigade, said firefighters could not reach the site due to the intensity of the explosions.

"None of the trucks has entered the camp as it's not safe to go inside. We have not handled this magnitude of armory fire," he told Reuters.

Since the end of a 26-year war against Tamil separatist guerrillas seven years ago, Sri Lanka's military has stored its weapons in a few armories around the country. Army spokesman Jayanath Jayaweera said the Salawa armory was one of the largest in the island nation.

The main road in front of the camp, which thousands of workers take to go to and from Colombo, has been closed to traffic. The government ordered all schools and government offices in the area to remain closed on Monday.