The government says civilians are being used as human shields by rebels
Sri Lanka's government should allow aid agencies back into the rebel-held north where fighting has displaced thousands, the Human Rights Watch group says.
Aid workers were ordered out of the area in September for their "safety".
The fate of civilians in the area has become a key issue as soldiers continue an offensive to crush the rebels.
A government spokesman indicated the order would not be reversed. He said food convoys and supplies of medicines were reaching rebel-held areas.
The World Food Programme said on Tuesday it had completed its largest food delivery to displaced people in the northern Vanni region since starting such convoys in October.
It said the delivery was the eighth to date and sufficient to feed more than 230,000 people for a week.
Human Rights Watch estimates as many as 300,000 people have fled their homes ahead of a military advance, moving further into rebel controlled territory.
Staff from the United Nations and other humanitarian agencies were ordered out in September when the government said it could not guarantee their safety.
Human Rights Watch is also calling on Sri Lanka's government to stop detaining the relatively few Tamil civilians - around 1,000, it says - who have fled the rebel area in recent months.
Rajiva Wijesinha, the human rights ministry secretary, dismissed the allegations.
"I think the real problem is [Human Rights Watch] have exaggerated this whole business. There is no question of detention," he told the BBC Sinhala service.
The government says Tamil civilians are being looked after in welfare centres, but the rights group says they are being held indefinitely in what amount to badly disguised prisons.
Earlier this month the organisation also detailed abuses being carried out by the Tamil Tigers, including preventing civilians leaving their territory and forcing people into their ranks to fight.
Sri Lanka's government says Tamil civilians are being held in the north as human shields. President Mahinda Rajapaksa has urged the rebels to release them.