A suspected Tamil Tiger front organisation has threatened to attack civilian targets including hospitals and water reservoirs in southern Sri Lanka in retaliation against military strikes on rebel areas.
The High Security Zone Residents' Liberation Force, which claimed responsibility for a rash of deadly attacks on troops in the north earlier this year, said it was giving the military a final warning to halt attacks on rebel territory.
The military and Liberation Tigers of Tamil Eelam (LTTE) each accuse the other of provoking near daily artillery and mortar bomb duels in the north and east.
"Purposeful destruction of Tamil civilian lives and infrastructure would lead to Sinhala civilian lives and infrastructure becoming inevitable targets," the suspected front said.
"We are sure that the people of the south are fully aware of the sort of humanitarian catastrophe they would have to face if one of the dams in the south is to burst," it added.
"Our retaliation may be at anywhere and at any time. We urge civilians and the international community to take due care."
The group referred to an air raid near a hospital in the rebels' northern stronghold of Kilinochchi on Thursday, an attack which killed five civilians in a nearby house and which the island's main donors -- Japan, Norway, the United States and the European Union -- have condemned.
"Every action has an equal and opposite reaction," the group said. "There are several hospitals in southern Sri Lanka. Not all are guarded by barbed wire and special forces."
It said it could also disrupt schools in the south if Tamil students' exams were delayed in the army-held Jaffna peninsula, which is cut off from the rest of the island by rebel lines. The group did not say elaborate.
The Tigers say suspected fronts like the high Security Zone Residents' Liberation Force have been formed by disgruntled Tamils who have decided to take up arms, and are not part of their movement.
Analysts said the threats should be taken seriously, and fear that the Tigers could turn to guerrilla attack tactics from the more conventional warfare which the foes have been fighting in the north and east since renewed civil war flared in late July.
"It is definitely the Tigers using another arm to set the ground for something they are planning," Iqbal Athas, an analyst with Jane's Defence Weekly, said on Monday.
"I would think an attack becomes inevitable in the current scenario," he added. "It is a frightening scenario."
The first peace talks between the foes in eight months collapsed a week ago over a rebel demand that the government reopen the main north-south highway, which runs through Tiger territory to Jaffna.