* Some 3,000 Palestinians sought shelter in U.N.-run school
* U.N. official says five Israeli tank shells hit school
* No immediate comment from Israeli military on incident
By Nidal al-Mughrabi
GAZA, July 30 (Reuters) - Israeli shells blasted a U.N.-run school in a Gaza refugee camp on Wednesday, killing at least 19 people and wounding about 125 others taking refuge there, a U.N. official said.
Blood splattered floors and mattresses inside classrooms, and some survivors picked through shattered glass and debris for flesh and body parts to bury.
At the edge of the schoolyard, some 20 donkeys lay dead, still tied to a railing.
An Israeli military spokeswoman in Tel Aviv said she had no immediate information about what had happened at the facility, which belongs to the United Nations Relief and Works Agency (UNRWA), which said it had recovered fragments from the shells.
Khalil al-Halabi, director of UNRWA's northern Gaza operations, said some 3,000 Palestinians were taking refuge in the school, in Jebalya refugee camp, when it came under fire around dawn.
"There were five shells - Israeli tank shells - which struck the people and killed many of them as they slept. Those people came to the school because it is a designated U.N. shelter," he said.
Five of the wounded were in critical condition, Halabi said.
Abdel-Karim al-Masamha, 27, said he and his family had come to the school after fleeing fighting near their home in the northern Gaza Strip.
"We did not find safety here," he said. "People were martyred before our eyes. They were dismembered."
Jebalya, which has been under Israeli fire for the past 24 hours, is the largest refugee camp in the Gaza Strip. Its population of 120,000 has been swollen by Palestinians trying to escape fighting between Israel and militant groups in the 23-day-old conflict.
"There is no safety at all, no place is safe, neither homes nor schools," said Haleema Ghabin, holding her infant son at the school. "What shall I say to the world? Find us a solution, we are defenceless civilians and children."
UNRWA, the main U.N. relief agency in Gaza, said it was at "breaking point" with more than 200,000 Palestinians sheltering in its buildings following calls by Israel for civilians to evacuate whole neighbourhoods before military operations.
Last Thursday, 15 Palestinians were killed and 200 injured in a strike on a U.N. school in the northern Gaza town of Beit Hanoun, crowded with hundreds of displaced civilians.
U.N. Secretary-General Ban Ki-moon condemned that attack and said he was appalled by the bombardment of a U.N.-designated shelter. The Israeli military said militants had fired from the vicinity of the school in Beit Hanoun and soldiers shot back.
Last week, Ban also expressed alarm at the discovery of rockets at two UNRWA schools. On Tuesday, rockets were found at a third, and a spokesman for the agency condemned "the group or groups who endangered civilians" by placing munitions there. (Editing by Jeffrey Heller in Jerusalem and Louise Ireland)