(CNN) -- A hospital caught in the middle of fierce fighting between government forces and Tamil rebels in northern Sri Lanka has been closed down after being shelled for a ninth time in four days.

A civilian, injured during fighting in rebel territory, lies on a bed at a hospital in Vavuniya on January 16, 2009.

A U.N. official says the facility, the last functioning hospital in the area, was fully evacuated Wednesday morning in the midst of the onslaught.

"Our staff has been dug down into bunkers for the past 18 hours under sustained artillery fire, which included cluster munitions this morning," said U.N. spokesman Gordon Weiss. "We hold grave fears for the safety of these [workers]. It also includes 81 staff members who are dug down in these bunkers."

The heaviest shelling on the hospital happened Sunday, when the facility was hit six times. Since then, Pudukkudiyiruppu hospital in the Vanni region has been hit once each on Monday, Tuesday and Wednesday, according to the ICRC's Sarasi Wijeratne in Colombo, Sri Lanka.

The shelling of the hospital has left at least 14 people dead and 10 wounded, the ICRC said.

Government forces and Tamil rebels are locked in a battle for the remaining rebel strongholds in northern Sri Lanka, where the country's ethnic Tamil minority has been fighting for an independent homeland since 1983. Each side accuses the other of firing on the hospital.

The Sri Lanka military told CNN on Monday that it knows the coordinates of the hospital and wouldn't have fired on it. The army also says the rebels are running low on ammunition and are firing shells indiscriminately in desperation, suggesting one of those could have hit the hospital.

A nurse at the hospital, meanwhile, accused the military of attacking civilians in a video provided by the Tamils Rehabilitation Organization, a group closely allied with the rebels. Watch video an aid group says shows civilians suffering »

Civilians "were attacked by the military, aerial bombing, and many have been deeply wounded and affected," said Dayani Ponnaiya, who identified herself as a nursing officer. "Many civilians have died, have lost hands and legs. Even children have been injured."

International humanitarian aid agencies have asked for increased access to the region, calling conditions in northern Sri Lanka a nightmarish situation.

The fighting has kept aid from reaching the hospital and has chased away doctors and nurses, Ponnaiya said, adding that many people have died as a result.

"People's lives have to be protected," she said. "Unless you are here to listen to their cries, you [won't] know their suffering."

Government officials, meanwhile, have accused 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 earlier this week.

Humanitarian groups say as many as 250,000 unprotected civilians are trapped in the area where the fighting is taking place.

Sri Lankan President Mahinda Rajapaksa has promised to allow safe passage to trapped civilians and urged the Tamil Tigers, as the rebels are commonly known, to promise the same.

All About Sri Lanka • Liberation Tigers of Tamil Eelam • International Federation of Red Cross and Red Crescent Societies