"The area is affected by shelling every day, and the cramped conditions and the lack of water and proper sanitation are putting people at risk," it said in a statement.
The Sri Lankan government estimates that 70,000 civilians are trapped in the island's northeast along with around 500 Tamil Tiger fighters.
The United Nations says the number of non-combatants could be about 200,000.
The military has surrounded the separatist guerrillas in a small patch of jungle and hopes to wipe out the rebels last fighters within weeks.