Both sides in the war are accused of ignoring the plight of civilians
Amnesty International has appealed to Sri Lanka's government to allow aid to reach more than 300,000 people displaced by fighting in the north.
The human rights group urged both the government and Tamil Tiger rebels to allow international monitors into the northern Wanni region.
There has been intense fighting in the area over recent weeks.
The rebels are fighting a decades-long war with the government to create a homeland for the Tamil community.
Amnesty says that thousands of people displaced by the conflict are currently "in desperate need of shelter, food and sanitation".
"As the north-east monsoon season approaches, only 2,100 temporary shelters for 4,000 families have been provided, leaving at least 20,000 families in need of protection from the elements," a statement released by the group says.
"Around two-thirds of the civilian population of the Wanni region have been forced out of their homes and are living in camps in areas controlled by Tamil Tiger rebels."
The human rights group says that unless food is distributed by aid agencies - rather than the government - "there is no way of establishing if the aid is reaching those most in need".
The civilian population is suffering, Amnesty says
Amnesty has also called on the Tamil Tigers "to ensure freedom of movement for civilians and stop preventing people from moving to safer places".
"The abuses carried out by the rebels make our call for international monitors to be allowed to visit conflict-affected areas even more pressing," the Amnesty statement said.
"These families must not be forgotten and left to suffer in a war zone."
Neither the government nor the rebels have responded to the latest report, but in the past both sides have denied they are responsible for the sufferings of the civilian population.
Meanwhile the government has announced that some 71 million rupees (about $645,000) - which belonged to a frozen bank account of the Tamil Rehabilitation Organisation (TRO) - had been "forfeited" to the state.
The government said that it was making the move because "large sums of money... purporting to be funds for humanitarian projects in Sri Lanka, were, according to intelligence reports, mainly used to finance terrorist activities".