A senior U.N. official called Thursday for “every measure” to be taken to protect civilians, following an attack in which at least 12 people were killed when tank shelling hit a U.N. training center hosting thousands of displaced Palestinians.

Thomas White, director of Gaza affairs for the U.N. agency UNRWA, said the situation in Khan Younis, the southern Gaza city where the attack took place, shows a “consistent failure to uphold the fundamental principles of humanitarian law.”

“Persistent attacks on civilian sites in Khan Younis are utterly unacceptable and must stop immediately,” White said. “People are being killed and injured. As fighting intensifies around hospitals and shelters hosting the displaced, people are trapped inside, and lifesaving operations are impeded.”

White said Wednesday’s attack also injured at least 75 people.

Israel’s military said it did not believe the shelling came from its forces but said the situation was under investigation. It also said the wider Khan Younis area was a significant base for Hamas militants.

The United States, which has been a key Israeli ally in its war against Hamas, condemned the latest assault that killed civilians, with the State Department saying, “We deplore today’s attack on the U.N.’s Khan Younis training center.”

State Department spokesperson Vedant Patel added, “You’ve heard me say it before, you’ve heard [Secretary of State Antony Blinken] say it before, but civilians must be protected, and the protected nature of U.N. facilities must be respected. And humanitarian workers must be protected so that they can continue providing civilians with the life-saving humanitarian assistance that they need.”

Philippe Lazzarini, head of UNRWA, said the compound had been clearly marked as a U.N. facility, and its coordinates had been shared with Israeli authorities.

Meanwhile, talks are continuing on a new 30-day cease-fire during which Israeli hostages held by Hamas and Palestinian prisoners held by Israel would be freed and more aid would enter Gaza, the narrow territory along the Mediterranean Sea.

U.S. special envoy Brett McGurk was in Doha, Qatar, to discuss the possibility of a halt in the fighting. A weeklong truce in late November led to the release of about 100 Israeli hostages and 240 Palestinians from Israeli detention.

The war started October 7 with the Hamas attack on southern Israel that killed about 1,200 people and the seizure of the hostages. Israel’s counteroffensive in Gaza has killed more than 25,700 people, according to the Hamas-controlled Gaza Health Ministry.

Some information for this report was provided by The Associated Press, Agence France-Presse and Reuters.