GENEVA (Reuters) - The U.N. humanitarian office (OCHA) said early on Friday that more than 400,000 people had fled their homes in the Gaza Strip and 23 aid workers had been killed since the start of Israeli retaliatory strikes in response to a deadly Hamas incursion.

The agency launched an appeal for nearly $294 million to help some 1.3 million people in Gaza and the West Bank, of which nearly half was programmed for food aid as supplies run out.

"Mass displacement continues. In the Gaza Strip, the cumulative number of internally displaced persons (IDPs) increased by 25% over the past 24 hours, now exceeding 423,000, of whom over two thirds are taking shelter in UNRWA schools," OCHA said, referring to the U.N. Palestinian refugee agency.

It said 23 aid workers had so far been killed since the weekend, including 11 health workers and 12 UNRWA employees.

(Reporting by Emma Farge, Editing by Friederike Heine)