COLOMBO, Sri Lanka - Patients are fleeing a hospital that was hit four times by artillery shells in the war between the Tamil Tiger rebels and government forces in northern Sri Lanka this week, the Red Cross said Tuesday.

Also Tuesday, the military said it had captured the rebels' seventh and final airstrip, effectively grounding their tiny air force as troops pushed ahead with their offensive to crush the guerrilla group and end Asia's longest-running civil war.

On Monday, independent observers handed The Associated Press dramatic pictures and video, showing scores of civilians killed or maimed. The images offered a rare glimpse of the growing toll the civil war is taking on the estimated 250,000 civilians trapped in the all-but-sealed conflict zone.

Even the wounded being treated at a hospital in Puthukkudiyiruppu were not safe. Artillery slammed into the hospital three times Sunday, killing nine patients and injuring 20.

It was hit again on Monday and three people were killed and 10 injured, said Sarasi Wijeratne, the spokeswoman of the International Committee of the Red Cross in Colombo.

The hospital is one of the last functioning medical facilities in the area where the Tamil Tigers are fighting a last-gasp battle for survival.

"People have started to leave the hospital because they don't feel safe there," said Wijeratne. "Those who are able to move are going away in search of safer places or to find shelter in less exposed locations."

The ICRC is negotiating with both sides to secure safe passage for the sick, and evacuations for the wounded, Wijeratne said.

"We need security assurances. Civilians have to be protected under international humanitarian laws," she said.

The civilians are trapped along with the rebels in a 300-square-kilometre sllice of jungle and rural area, surrounded by government forces.

The military says it is on the verge of destroying the Tamil Tigers, and ending their war for a separate homeland for the minority Tamil in the north and the east that started in 1983 and has cost more than 70,000 lives.

The military said it captured a runway on the edge of what remains of the rebels' once-substantial de facto state in the north. Troops had captured six other runways in fighting in recent months. However, they have not found the three or four aircraft that comprise the rebel air force.

The rebels have used their tiny air wing to carry out a series of attacks on Sri Lankan military bases and the capital, Colombo, badly embarrassing the Sri Lankan government. In one attack in 2007, the rebel airplanes bombed the country's only international airport, creating panic among travellers.

Information from the war zone is impossible to verify, since journalists and most aid groups have been barred from the area. But independent observers shot video footage and photographs over the past week and provided them to the AP on condition they not be identified because they feared government reprisal.

One photograph, taken Jan. 23, shows a mother and father dead on the floor, their two young children cradled between them, also dead.

They were apparently killed in their sleep when an artillery shell hit their makeshift shelter in the village of Udayarkattu, according to the observer who took the picture.

The village is inside the "safe zone" that the government established Jan. 21 inside rebel territory as a refuge for civilians.

The government pledged not to attack the safe area, but local officials and human rights groups say it has come under repeated artillery attack.

The video footage shows the Puthukkudiyiruppu hospital before it came under the artillery barrage.

It shows young boys and girls with amputated legs and arms, and an elderly woman missing her right leg writhing on a mat on the floor of the hospital. A toddler, his head bandaged and left eye swollen closed, lay nearby, his gauze-covered hands useless as flies buzzed around his face.

"We were caught in shelling after I unloaded our goods. Both my sisters were killed," a teenage boy with no arms sobs on the tape.

Asked about the video and photographs, military spokesman Brig. Udaya Nanayakkara asserted: "There may be civilians injured, but not due to shelling. They may be injured because they have been employed on the construction of (rebel) defenses. Civilians maybe have been injured due to crossfire."

Dr. Thurairajah Varatharajah, the top health official in the war zone, estimated last week that more than 300 civilians had been killed in the recent fighting, something the government has denied. Varatharajah has not updated his estimate.

The government accuses the rebels of holding the civilians against their will as human shields, a charge the rebels deny.