The United States has urged Sri Lanka's army and Tamil Tiger rebels to avoid indiscriminate shooting in the no-fire zone, where tens of thousands of civilians remain trapped.
State Department spokesman Robert Wood said there are credible reports of rising civilian casualties. He also said there have been incidents of rebels firing on civilians as they try to flee the shrinking war zone.
Separately, the United Nations said it is stepping up efforts to help civilians who have fled the fighting. A U.N. spokeswoman, Marie Okabe, said food aid through the Office for the Coordination of Humanitarian Affairs is well funded, but that shelter, water, health and sanitation needs are not.
The U.N. World Food Program said it has enough food to feed 100,000 people for the next two weeks.
The UN has accused the rebels of using civilians as human shields and preventing them from leaving the war zone. The world body has also said the Sri Lankan army is firing into so-called safe areas.
Each side denies targeting civilians.
Other aid agencies have issued warnings, too.
On Tuesday, the International Committee of the Red Cross called the situation "catastrophic." Human Rights Watch has urged rebels and the army to show greater concern for civilians.
And the International Crisis Group has called the situation unfolding in Sri Lanka a "humanitarian tragedy."
Some information for this report was provided by AFP, AP and Reuters.