"The level of human losses and destruction in Gaza is really immense," said UNWRA spokesman Sami Mshasha.
"According to our latest figure, we are talking about 174 killed and well over 1,100 injured. This number will increase. The numbers are increasing by hours," he told reporters.
"A good number of those killed and injured are women and children. That is a cause of concern for UNRWA," he added.
The death toll on Tuesday had risen to 192, according to local officials in Gaza.
Mshasha said that 560 homes had been totally destroyed, while thousands of buildings had suffered damage.
Mshasha said that 47 UNRWA facilities had also been damaged by bombing.
A total of 17,000 people had found refuge in 20 schools run by the UN agency, which has sent their GPS coordinates to Israeli authorities.
He called on the warring sides to respect UN buildings.
Israel on Tuesday announced it had accepted an Egyptian ceasefire plan after a week of bombing, but the move was rejected by the Palestinian Islamist movement Hamas which controls the territory and has fired a hail of rockets at Israel.
Without a halt to the fighting, Mshasha said, the risk of an Israeli ground assault on the enclave remains, raising the specter of even greater death and destruction.
In a separate statement, the International Committee of the Red Cross said the bombing had devastated Gaza's water supply.
"Hundreds of thousands of people in Gaza are now without water. Within days, the entire population of the Strip may be desperately short of water," said Jacques de Maio, who heads ICRC operations in Israel and the Palestinian territories.
"Water and electrical services are also affected as a result of the current hostilities. If they do not stop, the question is not if but when an already beleaguered population will face an acute water crisis," he said in a statement.
Fighting is also preventing technicians from carrying out essential repairs.
Following the deaths of several municipal water technicians, Gaza's water service provider has suspended all field operations until the safety of its staff can be guaranteed, the ICRC said.
"Gaza's water system has been deteriorating for years. The latest attacks are the last straw. Safe drinking water is becoming increasingly scarce in the Strip, just as temperatures are soaring," said ICRC water and sanitation expert Guillaume Pierrehumbert.
"Water is becoming contaminated and sewage is overflowing, bringing a serious risk of disease."