Sri Lanka today warned of a "serious security threat" to its electricity grid as engineers battled for the fifth straight day to restore supplies crippled by an explosion and fire, AFP reported.

Deputy Minister of Power and Renewable Energy Ajith P. Perera said investigators had not ruled out sabotage in Sunday's blast and a fire at the main distribution centre just outside the capital.

"There is a serious security threat. That is why the military has been called in to protect these (electricity) locations," the minister told reporters in Colombo.

He said investigators had so far been unable to determine the cause of the blast that plunged the country into darkness for about eight hours, the worst black-out in 20 years.

Authorities imposed electricity rationing of seven-and-a-half hours daily as engineers tried to restore normal supplies after Sunday's fire.

But by Thursday afternoon, the authorities had still not managed to get the system fully back online.

Perera said only one of three generators at the Chinese-built coal-fired power plant that automatically shut down on Sunday had been reconnected.

The $1.35 billion plant commissioned in 2011 has been plagued with frequent breakdowns, he said.

Sunday's was the second major power failure in less than a month and the worst since May 1996, when the entire country was without electricity for four days.

(With inputs from AFP)