(CNN) -- International humanitarian aid agencies called Monday for increased access to the "nightmarish situation" in northern Sri Lanka, where government forces and Tamil rebels are locked in battle. Hundreds of thousands of civilians are thought to be trapped by the fighting.
A civilian, injured during fighting in rebel territory, lies on a bed at a hospital in Vavuniya on January 16, 2009.
The plea came a day after artillery shells slammed into a hospital in the northern district of Mullaittivu, where civilians -- including a growing number of children -- are being treated.
Sarah Crowe of UNICEF said aid from the United Nations is getting into the war zone only every few days.
"We need open access," she said. "These children and families need to be protected and they need to get out fast."
More than 200 civilians, including at least 30 children, had been injured in the past three days of fighting, a relief worker told CNN Sunday.
"That is the absolute minimum [number of injured]," the relief worker said. The person did not want to be identified for fear of jeopardizing the work of relief organizations.
Government officials are accusing aid organizations and foreign media of sensationalizing civilian casualties.
"It looks as if it's convenient for certain agencies to exaggerate the numbers so that this can be converted to a humanitarian crisis in the public eye, " Secretary of Foreign Affairs Dr. Palitha Kohona told CNN. 
Watch video an aid group says shows civilians suffering »
On Sunday, Defense Secretary Gotabaya Rajapaksa promised to "continue with the military offensive until we liberate the remaining area under LTTE [the rebel group's] control," according to Sri Lanka's state-run news agency.
A handful of U.N. staff are working round the clock to save a growing number of children caught in the crossfire, a U.N. spokesman said Saturday. Watch Sri Lanka's U.N. ambassador speak on the conflict »
Children as young as 4 months old were being treated in area hospitals for shrapnel injuries and other wounds of war, spokesman James Elder told CNN.
"There is just intense fighting in a small area where children and other civilians are," Elder said. "The space [where conflict is taking place] is shrinking and the fighting is augmenting."
Thursday, U.N. aid workers rescued 50 critically injured children and 105 adults, he said.
"We are trying to get as many people out of there as we can," Elder said.
Humanitarian groups say as many as 250,000 unprotected civilians are trapped in the area.
"You can see on people's faces, they're full of shock, fear -- they're scared," Lisabeth List, a nurse with Doctors Without Borders, told CNN Monday. "They don't know what will happen to them and they're concerned about their families who they had to leave behind."
Sri Lankan President Mahinda Rajapaksa has promised to allow safe passage to trapped civilians and urged the Tamil Tigers to promise the same.
"We have declared a safe zone for civilians, the coordinates of which were announced by the security forces," Rajapaksa said on his government's Web site.
"It is unfortunate that the [Tamil Tiger group] is exploiting this declared safe zone for civilians by placing their heavy artillery within the safe zone and using it as a launching pad to attack security forces and indiscriminately kill civilians."
The fighting has created a "nightmarish" situation for civilians in the conflict zone, Elder said.
An emerging shortage of humanitarian supplies and diminished access to clean water, sanitation and food are compounding a crisis, he said.
On Sunday, Sri Lankan soldiers seized a key rebel stronghold in a surprise attack deep in Tamil-held territory.
Troops crossed a lagoon and entered the town of Mullaittivu before encountering heavy resistance from Tamil fighters, according to the government-run news agency.
The Liberation Tigers of Tamil Eelam (LTTE) -- commonly known as the Tamil Tigers -- have fought for an independent homeland for the country's ethnic Tamil minority since 1983. The civil war has left more than 70,000 people dead.
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