UNITED NATIONS - Six staff members of the UN relief agency for Palestinians, known as UNRWA, were killed in Israeli airstrikes in Gaza on Wednesday, UN Secretary-General Antonio Guterres said.

A school-turned-shelter for around 12,000 people was hit by Israeli airstrikes on Wednesday, and six UNRWA staff members were among those killed, Guterres said on X, formerly Twitter.

"What's happening in Gaza is totally unacceptable," he said. "These dramatic violations of international humanitarian law need to stop now."

At least 18 Palestinians were killed and many others wounded in an Israeli airstrike on a UN-run school sheltering displaced people in the central Gaza Strip on Wednesday, according to Palestinian sources.

The sources told Xinhua that an Israeli warplane fired at least one missile at the shelter in the al-Nuseirat refugee camp. The Hamas-run Gaza government media office said that aid workers were among the victims.