* Over 50,000 flee violence to U.N. shelters
* UNRWA warns supplies running critically low
* Hospitals under strain as deaths and injuries rise
By Noah Browning
BEIT LAHIYA, Gaza Strip, July 19 (Reuters) - In a school in northern Gaza, scores of families sweat in cramped classrooms. Babies cry, while restless kids draw on chalkboards and worried parents give thanks for their relative safety.
They are among more than 50,000 civilians taken in by the main United Nations agency in Gaza, UNRWA, as they flee heavy shelling amid an Israeli ground offensive into the border areas of the Gaza Strip.
UNRWA is now warning that its funds and supplies are running critically low. From Sunday, it says, it will no longer be able to provide enough mattresses for the flow of refugees, and has launched an urgent appeal for 60 million dollars.
"The number of those fleeing continues to rise inexorably and has more than doubled in the last 36 hours," UNRWA spokesman Chris Gunness told Reuters.
At least 325 Palestinians, including 70 children, have been killed in the 12-day cross-border battle between Israel and Palestinians militants, according to Gaza officials. An Israeli soldier and civilian have also been killed.
Israel says its attacks target militants, whom they say use civilians as "human shields." Its ground operation aims to raze border tunnels from which guerrillas hope to attack Israel.
Hundreds of Palestinian rockets have rained down on Israeli cities as Israel has carried out its air and ground campaign, pledging to put an end to the salvos and protect its people.
Israel has made calls and tapped local TV channels to broadcast warnings to residents of border areas to escape.
"My family and I left the house with nothing but the clothes we were wearing at two in the morning. No car was around to take us - we walked in the total darkness and arrived here, thank god," said Kareem Ramadan, 45, an unemployed father of three.
"Maybe we'll go back and see the house destroyed, but at least we're alive.
Some 2,200 Palestinians have been wounded since the fighting began. In response doctors are working 24-hour shifts every other day and hospitals, which rely on generators for power in the electricity-starved Strip, have heavily pared back treatment for regular patients.
Staff at Kamal Adwan hospital don't flinch at the nearby crashes of artillery fire - fighting has become routine in the coastal territory of 1.8 million - but they are still sometimes surprised.
Doctor Tawfeeq Ahmed was on duty when his wife, 19-year old daughter and 5-year old son were whisked into the hospital after an Israeli shell landed on his house, slicing them with shrapnel but leaving no serious injuries.
"When I left them they were just sitting in the living room. Then they show up here, and in this condition. It was such a shock," he said.
Elsewhere in the hospital 25-year old policeman Rashad, stationed on duty, said the grave injuries and dead bodies rolling in no longer had much impact on him.
"But I do get affected when I rush out with the ambulances and I see houses of normal people in chaos, destroyed," he added. "It makes me think, we're all victims in these wars."
While the doctors work and security staff keep a look out, other employees scrape soapy water across the pavement in front of its trauma ward, hoping to wipe clean the blood spatters from the flow of victims from the Beit Lahiya area near the northern border.
The spots don't come out. (Editing by Sophie Walker)