Sri Lanka's army says it has bombed a sea base from where Tamil Tiger rebels launched attacks on government positions in the Jaffna peninsula.
Sri Lanka has seen fierce clashes in the last week
A rebel website said the facility was a civilian shipyard and that bombs had injured four civilians.
Denying the report, the military said it had destroyed many rebel boats.
Aid agencies have warned that the northern Jaffna peninsula is cut off by fighting and aid cannot reach thousands of trapped civilians.
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 while it is unclear how many rebels have died.
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 office of the UN High Commissioner for Refugees in Geneva said this week that the key access road to the northern Jaffna peninsula through a rebel-held district was closed.
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
"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.
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.