Heavy fighting has broken out between two Tamil Tiger rebel factions in north-eastern Sri Lanka.
The mainstream Tigers have threatened to kill Colonel Karuna
Members of the main Tiger movement exchanged fire across the Verugal river with fighters controlled by a breakaway commander, Colonel Karuna.
At least nine fighters are thought to have been killed and another nine people injured.
The International Committee of the Red Cross said thousands of civilians were fleeing from the area.
The battle comes days after a new government took office and is the first between the rival rebel factions.
De facto border
The casualty figures come from rebel sources but only relate to the south of the area affected, not to the north, so the figures may be higher.
The BBC's Frances Harrison in Colombo says it is not clear whether this is a one-off bout of fighting or the beginning of a major clash between the two factions.
Fighting is thought to have taken place close to the coast.
The TamilNet website, which is close to the main faction in the north, says infantry formations spearheaded by crack commando units and backed by heavy artillery advanced south across the Verugal river that formed a de facto border between the two rebel groups.
TamilNet said: "More than 300 young fighters of the Karuna group who were defending the southern side of the [Verugal] river surrendered without a fight."
That sparked concern at the United Nations Children's Fund, Unicef, which said any underage fighters should immediately be released into its care.
The head of Unicef in Sri Lanka, Ted Chaiban, said both sides should desist from using anyone under 18 to fight and should keep them out of harm's way.
Television pictures from eastern Sri Lanka showed two injured male child soldiers in hospital who had fought for Colonel Karuna.
Regional officials said fighters fired mortars and small arms fire for nearly two hours across the river.
A spokesman for Colonel Karuna did say the northern forces had penetrated its territory, which would involve crossing the river.
But it is not clear how far they have advanced.
Both sides have been building up their forces
Military officials say they believe there is now a lull in the fighting during daylight hours.
A spokesman for the international monitors of a two-year ceasefire between the government and Tiger rebels said the northern faction had captured a small coastal base at Kathiraveli, some way into territory controlled by Colonel Karuna.
The monitors said northern rebels crossed the river and at the same time conducted a beach landing in a pincer movement.
Technically, any sea movement like this would constitute a violation of the ceasefire.
The monitors have met defence officials whom they say are extremely concerned about the situation.
They have also talked to the northern faction which confirmed there had been fighting in an attempt to regain control of the eastern forces.
The district medical officer at Valaichchenai in eastern Sri Lanka says two dead female fighters were brought to his hospital and then removed by rebels loyal to Colonel Karuna.
A doctor in the town said an ambulance driver had been beaten by rebels loyal to Colonel Karuna and forced to go to pick up casualties.
On the way, the ambulance hit a mine and the driver was killed.
Another doctor said he had been threatened on the telephone by one faction and told not to treat the injured of the other.
The rebel organisation split last month after Colonel Karuna, a senior commander in the east, broke away, saying he wanted to form his own administration.
The mainstream Tigers have denounced Colonel Karuna as a traitor and threatened to kill him.
Both sides have been building up troops on the river dividing their territories.