(CNN) -- Hundreds of civilians died during weekend fighting between Sri Lankan government forces and Tamil rebel fighters, the United Nations reported Monday.
A photo supplied by a humanitarian group on Sunday shows civilians allegedly injured in government shelling.
"Verification is, of course, the part of the problem, because humanitarian agencies are denied access to the region," U.N. spokesman Gordon Weiss said from the capital, Colombo.
"Our understanding is that very heavy fighting took place over the weekend, and there were many hundreds of civilian deaths and many, many more beyond that injured."
The numbers provided by the United Nations pale in comparison with rebel claims. More than 3,200 civilians died from Sunday evening till Monday morning, said Lawrence Christy, the head of Tamils Rehabilitation Organization, according to the rebel Web site TamilNet.
The "Sri Lanka Army (SLA) has stepped up relentless barrage of shelling using all sorts of heavy weapons on the so-called safety-zone (no-fire zone) ... areas where more than 130,000 civilians have sought refuge," TamilNet said.
The Sri Lankan government denied the allegations and accused the Tamils of "taking the whole world for a ride with its well-engineered propaganda ploys," while turning their weapons on civilians trying to flee the combat zone.
Accounts vary widely about how many have been trapped. As recently as last week, a military spokesman put the number at 10,000 to 15,000 civilians.
CNN could not independently verify the accounts, because the government does not allow the news media independent access to areas where it is fighting the rebels.
Government troops have closed in on rebel forces in a shrinking section of the country's north, and civilians have been caught in crossfire.
More than 196,000 people have fled the battle zone, according to the U.N. Office for the Coordination of Humanitarian Affairs.
Tamil Tigers rebels have been fighting for an independent state in Sri Lanka's northeast since 1983.
As many as 70,000 people have been killed since the civil war began, and the Tigers have been declared a terrorist organization by the European Union and more than 30 countries, including the United States.