Sri Lanka's military said there were reports of a large explosion at sea just north of the capital on Friday, but details remained sketchy and at least two defence officials said it might be a sonic boom from a jet.
The military said the blast appeared to have occurred offshore some 10 kms north of Colombo towards the island's main international airport -- the target of a 2001 suspected Tamil Tiger suicide attack.
"It's somewhere at sea. No details yet. We have sent air force and navy to the scene," a defence spokesman said.
But air force spokesman Group Captain Ajantha de Silva said it appeared to be a false alarm caused by a government jet.
"It's a sonic boom," he said.
On Saturday, police said they arrested three suspected Tamil Tiger rebel frogmen on a beach nearby.
The government said they were seized with explosives and another government spokesman said the blast could be from one of them.
With violence rising and a 2002 ceasefire seen near collapse, the country is extremely tense.
So far, violence has largely been restricted to the island's north and east but many fear attacks in the capital.