Israeli tanks struck a huge UN compound sheltering displaced Palestinians on Wednesday, causing “mass casualties”, the UN said, as Israeli forces battled their way into crowded parts of Gaza without letting people out first.

The attack, which the UN said hit a vocational training compound housing 30,000 displaced people in Khan Younis, southern Gaza’s main city, prompted rare outright condemnation from the US of an apparent Israeli military action.

“Mass casualties have taken place, some buildings are ablaze and there are reports of deaths. Many people are trying to flee the scene, but unable to do so,” UN humanitarian coordinator for the Palestinian territory, James McGoldrick, said.

Thomas White, director of Gaza affairs for the UN agency, UNRWA, said at least 12 people were killed and 75 wounded in a building housing 800. The agency’s head Philippe Lazzarini, reposting that statement, said the death toll was probably higher.

“The compound is a clearly marked UN facility and its coordinates were shared with Israeli Authorities as we do for all our facilities. Once again a blatant disregard of basic rules of war,” Lazzarini said.

In Washington, the US state department spokesperson Vedant Patel said: “We deplore today’s attack on the UN’s Khan Younis training centre.” “Civilians must be protected, and the protected nature of UN 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,” Patel said.

Washington has raised concerns and asked Israel for information over incidents during the conflict, but has rarely been openly critical of a specific Israeli action.

Hours after the attack as night fell, UN staff were still unable to reach the area and all communications were shut down.

Israeli forces have launched their biggest ground offensive in at least a month, encircling the city which is housing hundreds of thousands of people who fled fighting elsewhere in Gaza. Residents say that unlike in previous Israeli assaults, announcements warning them to leave came only after the operation was underway and the main road out already shut.

The bulk of the 2.3 million-strong population of Gaza is now penned into Khan Younis and the towns just north and south of it. Palestinian officials say the Israelis have cut off and besieged the city’s main hospitals, making it impossible for rescuers to reach many of the wounded.