Tamil Tiger rebels say 165,000 civilians still living in the small area in the north-east of Sri Lanka that is under their control, are nearly out of food, and compare the situation to the crisis in Darfur.
Amid international concern for people trapped in the war zone, the Liberation Tigers of Tamil Eelam, or LTTE, said in a statement that starvation among innocent people was "imminent" and accused the government of blocking supplies.
The Tigers, facing defeat after nearly four decades of fighting, said the military has blocked land routes leading to the rebel areas, leaving sea transport the only option to deliver food and medicine.
"We fear that further delay can result in a crisis similar to that faced in Darfur or even deadlier," the LTTE said, urging the United Nations and the international community to ensure uninterrupted supplies to the region.
It is not clear how many people are trapped in the small rebel-held area, with the UN putting the number at 50,000 and the government at 15,000-20,000.
The Tigers said they welcomed the scheduled visit on Saturday of UN rights chief John Holmes to Sri Lanka.
"We are prepared to engage in a constructive dialogue to address the humanitarian crisis," the rebels said.