The Tamil Tigers are fighting for independence for the island's minority 2.5 million Tamil community in the majority-Sinhalese nation of 19.
5 million people, where more than 3,500 people have been killed in the past year.
The bitter ethnic conflict has claimed more than 60,000 lives since 1972.
The Norwegian-led Sri Lanka Monitoring Mission (SLMM) accused the rebels of preventing civilians from moving out of the battle zone and blamed the military for preventing monitors from getting to the area to evaluate the situation.
"The SLMM has not been able to monitor the situation as well as it would have wanted as monitors have been continuously refused access by the Sri Lanka army into the areas of concern," the monitors said in a statement.
It said both the Liberation Tigers of Tamil Eelam (LTTE) and the military were in breach of the February 2002 truce with their conduct in the Vakarai area where heavy shelling continues since the weekend.
"The SLMM urges the parties to do their