The low-flying plane dropped two bombs on the facility in Kerawalapitiya, 6.5 miles north of Colombo, said a military official who witnessed the attack. He spoke on condition of anonymity because he is not authorized to talk to the media.
The extent of damage to the gas storage facility was not immediately known.
The government did not officially confirm the attack other than saying that the air defense system was activated and troops fired anti-aircraft guns.
"We are doing some air missions, we are investigating ," Air Force spokesman Group Capt. Ajantha Silva said.
The country's only international airport, an adjoining air force base and the entire capital were blacked out. Many people were awake at the time watching Sri Lanka's cricket team play Australia in the World Cup final on television.
All passengers aboard planes were called back into the terminals and air traffic was suspended for about an hour, an employee of the international airport said, speaking on condition on anonymity.
Last Tuesday, Tamil Tiger rebels used their new air wing to bomb military positions in the northern Jaffna peninsula, killing six soldiers. But the planes could not reach their intended target — a key military base that serves as the operational headquarters against the guerrillas.
A month before, the Tigers carried out their first-ever airstrike on an air force base near the capital, killing at least three airmen.
Sri Lanka's separatist conflict flared in 1983 when separatist rebels started fighting for an independent homeland for the country's 3.1 million ethnic minority Tamils.
More than 69,000 people have been killed in the conflict.