Aid workers need urgent access to parts of northern and eastern Sri Lanka that have been cut off by recent fighting, the UN's refugee agency has said.
Aid workers have been locked out of several areas in Sri Lanka
Thousands of people are suffering from food and water shortages because of road blocks and fierce clashes.
Sri Lankan officials opened access into the Tamil Tiger rebel stronghold of Kilinochchi for two hours on Friday.
It was the first time since fighting erupted in the north a week ago that access to the area had been re-opened.
The government allowed people and relief supplies to go in, but no-one was allowed to leave, the BBC's Dumeetha Luthra reports.
North of Kilinochchi, the Jaffna peninsula remains cut off, she added.
Intense fighting has been going on in northern Sri Lanka for the past week. The Sri Lankan authorities say at least 100 soldiers have died in the fighting. It is unclear how many rebels have died.
The office of the UN High Commissioner for Refugees in Geneva said the key access road to the northern Jaffna peninsula through a rebel-held district was closed.
"We call on the Sri Lankan government and the rebel Tamil Tigers to urgently allow access for humanitarian aid workers so vital supplies can reach those in need," said UNHCR spokeswoman Jennifer Pagonis.
Food and water supplies had fallen to "alarmingly low levels" in many areas, the agency said.
Up to 20,000 displaced people in Kilinochchi District
Displaced civilians in Jaffna and Point Pedro in the far north
Supplies of food and water "alarmingly low" in some areas
162,200 people have fled their homes since fighting began to flare up in April
The Sri Lankan government's most senior representatives in government-held parts of Jaffna and Kilinochchi have told officials in the capital, Colombo, that there are severe food shortages.
They have requested that at least 5,000 tons of essential food items be sent to each area immediately.
Sivanathan Kishore, an MP of the Tamil National Alliance party, confirmed the reports of shortages.
"Normally, 75 to 80 lorries go through this area. Now, because of the closure of the road, the food situation will definitely worsen in Vavuniya" in northern Sri Lanka, Mr Kishore told the BBC.
The government in Colombo has said it is sending 80 tons of aid to the Jaffna peninsula.
The aid will leave Colombo by boat on Saturday and take 55 hours to reach Jaffna, government spokesman Keheliya Rambukwella told the Associated Press news agency.
Sri Lanka's undeclared war is being conducted on three fronts, with air raids, artillery strikes and mortar attacks. About 100,000 people have now been affected by three weeks of hostilities.
The Jaffna peninsula has become the centre of the clashes.
Some 500,000 civilians in the area are living under a government-imposed 22-hour curfew.
Meanwhile, the Tamil Tigers on Friday gave the names of 51 schoolchildren they claim were killed in an air force bombing of an orphanage in rebel-held territory.
The rebels said 51 out of 55 people killed in Monday's air raid were children.
They had earlier said that 61 schoolgirls died in the bombing. There was no immediate explanation for the discrepancy.
The government has said it targeted a Tamil Tiger training camp in the attack and has denied killing any children.