Condemnation mounted on Thursday of a deadly Israeli strike on a school turned shelter in central Gaza, but Israel said that the compound, crowded with people driven from their homes, had become a command center for militant fighters.

The site, Al-Jaouni School, had been home to around 12,000 displaced people from the Gaza Strip, mainly women and children, according to the United Nations agency that operated the school. Israel has struck the compound five separate times since the war began last October, the agency said.

The Palestinian authorities said the Israeli strike on Wednesday killed 18 Gazans, including women and children, and injured a similar number. The primary U.N. relief agency for Palestinians, known as UNRWA, said six of its employees, including the shelter’s manager, were among the dead — the most UNRWA employees to die in a single strike in a war that has killed more than 200 of them.

Britain’s foreign secretary, David Lammy, on Thursday added his voice to the criticism from the United Nations and others, calling the deaths of the U.N. workers “appalling” and reiterating calls for a cease-fire between Hamas and Israel. The government of Qatar, a key mediator in talks over a cease-fire, called the strike a “horrifying massacre.”

The Israeli military defended the strike, which it said killed nine members of Hamas’s military wing. It said the compound in Nuseirat was being used as a Hamas “command and control center,” a claim it has made repeatedly to justify increasingly frequent strikes on schools serving as shelters.

Israel issued a list of nine names of people it said were Hamas militants who had been killed in the attack, including three who it said were employees of UNRWA.