The Red Cross says it's not able deliver aid to eastern Sri Lanka or evacuate civilians from the area held by Tamil Tiger rebels, because of heavy fighting.
More than 400 people were killed and over 1000 were injured over the weekend in what the United Nations has described as a bloodbath.
Many of those sick or wounded were supposed to have been evacuated by the International Committee of the Red Cross on Tuesday.
But the Red Cross says an aid ship that was waiting off the coast has gone back to its base.
A spokesperson has urged both the Sri Lankan army and the Tamil Tiger rebels to let aid agencies operate safely.
The Tamil Tigers claim government forces have shelled a makeshift hospital in the conflict zone but the government denies the army has caused civilian casualties or used heavy weapons in civilian areas.
The rebels are cornered in a coastal strip covering 2.5 square km. More than 116,000 civilians fled after troops broke a rebel barricade in late April.