Tamil Tiger rebels have said that Sri Lanka's four-year ceasefire is now void and the island's two-decade civil war back on as guerrillas and the military entered a sixth straight day of fighting.

S. Elilan, the head of the Tigers' political wing in the restive eastern district of Trincomalee, said on Monday, "The war is on and we are ready.

"The war has begun. It is the government which has started the war... The ceasefire agreement has become null and void."

On the ground, Tamil rebels fired artillery at troops in northern Sri Lanka on Monday, opening a new front in clashes as the army pushed toward rebel territory in the northeast, the military said.

Elilan confirmed the fighting, saying the government forces were continuing an advance towards their forward defence line in the east in a water supply dispute.

He said, "Militarily we have decided to fight back if the Sri Lankan army enters our area."

The independent Daily Mirror quoted the rebels as saying on Monday that the government offensive in northeastern Trincomalee had forced 3,000 Tamil residents to flee the area.

Accusation

The military said the Tigers opened fire at troops in Nagarkovil, which falls on the front lines between rebel and government held areas in the northern peninsula of Jaffna, causing no casualties.

Brigadier Prasad Samarasinghe, the military spokesman, said: "The terrorists fired at our troops in Nagarkovil in Jaffna and we retaliated."

Separately, Samarasinghe said government troops were making little progress in trying to reach a site where the guerrillas were blocking the flow of water from a plant to villages in government-held territory.

Keheliya Rambukwella, the chief government spokesman, said the troops were hampered by landmines and because the insurgents had positioned themselves on high ground.

On Sunday, air force planes bombed rebel positions in the area of the water blockade after government troops came under rebel mortar fire, Rambukwella said.

Under the terms of the cease-fire, the government and rebels must seek permission before entering each other's areas.

Months of violence, including clashes between government forces and the rebels, have killed about 800 people and battered the fragile 2002 cease-fire, which was meant to end two decades of fighting that claimed the lives of about 65,000 people.

Sri Lankan government soldiers arrive following a landmine blast in Kabithigollewa, about 210 kilometers (131 miles) northeast of Colombo, Sri Lanka, in this June 15, 2006 file photo.

PHOTO CAPTION

Sri Lankan government soldiers arrive following a landmine blast in Kabithigollewa, about 210 kilometers (131 miles) northeast of Colombo, Sri Lanka, in this June 15, 2006 file photo. (AP)