COLOMBO, Sri Lanka - Doctors and aides abandoned the only hospital in Sri Lanka's war zone amid unrelenting shell attacks, a health official said Thursday. The military said thousands of civilians braved rebel gunfire and fled across the front lines.

The medical staff was huddling in a nearby bunker because of the non-stop shelling, and could hear the cries of hundreds of patients unable to leave the hospital begging for food and water, according to a health official in the war zone.

The Red Cross said the tiny strip of coastal land still controlled by the rebels was wracked by violence, despite international appeals for the two sides to end the war and let the estimated 50,000 civilians trapped in the area escape.

Though the number of wounded was rapidly increasing, the medical staff fled the makeshift hospital in the region after it was shelled twice this week in attacks that killed about 100 people, the health official said, speaking on condition of anonymity because he was not authorized to talk to the media. Other attacks last weekend killed about 1,000 people in the tiny conflict zone.

About 400 badly wounded patients remained in the facility Thursday in desperate need of treatment, along with more than 100 bodies waiting to be buried, the official said. Many other wounded civilians fled after the second shell attack hit the hospital Wednesday, he said.

As the military pressed ahead with its offensive, 2,400 civilians waded across a vast lagoon that serves as a barrier between government and rebel forces under a hail of rebel gunfire to escape the area, military spokesman Brig. Udaya Nanayakkara said.

Most of the civilians managed to escape, though four were killed and 14 others wounded, he said. Another 2,000 civilians were on the far shore, still waiting to cross the lagoon, he said.

The rebels have denied accusations they were holding the civilians as human shields and shooting at those trying to flee.

The government has cornered the Tamil Tigers on the strip of land - bordered by the sea on one side and a vast lagoon on the other - and vowed to end the 25-year-old conflict.

With the death toll from the fighting mounting, U.S. President Barack Obama on Wednesday demanded the rebels lay down their arms and release those trapped. He also admonished the government to stop firing artillery into the war zone.

"Now's the time, I believe, to put aside some of the political issues that are involved and to put the lives of the men and women and children who are innocently caught in the crossfire, to put them first," Obama said in Washington.

The United Nations Security Council issued similar demands to both sides and expressed grave concern at the worsening humanitarian crisis.

Both sides welcomed Obama's appeal for an end to the civil war, but ignored his criticism of their conduct.

Sri Lankan Foreign Secretary Palitha Kohona said the government was "extremely reassured" by Obama's call for the rebels to lay down their weapons and release the civilians, and he denied the government was firing artillery into the densely packed war zone.

The rebels' political chief, Balasingham Nadesan, praised Obama in a statement emailed to The Associated Press "for passionately talking about the plight of Tamil civilians and calling for urgent actions to alleviate the mounting humanitarian crisis."

The rebels did not address his call for their surrender.