WASHINGTON -- The United States declared Sunday it is "appalled" by the "disgraceful" shelling by Israel of a United Nations school sheltering some 3,000 displaced people in southern Gaza.

In language that was rare in its directness and severity, the U.S. denounced the attack that killed 10 people earlier in the day, noting that the school had been designated a protected location.

"The co-ordinates of the school, like all UN facilities in Gaza, have been repeatedly communicated to the Israel Defence Forces," State Department spokeswoman Jen Psaki said in the statement. "We once again stress that Israel must do more to meet its own standards and avoid civilian casualties."

The U.S. condemnation follows one by U.N. Secretary-General Ban Ki-moon, who depicted the shelling near the Rafah school as both "a moral outrage and a criminal act."

Earlier, a senior Palestinian diplomat expressed outrage over killings and bloodshed on both sides in Gaza and called for negotiations to end the savage fighting that has gone on for nearly a month. "What we need now is to stop this fighting, to address the tragic humanitarian situation in the Gaza Strip," Riyad Mansour, the Palestinian ambassador to the U.N., said on NBC's "Meet the Press," adding "these things need to be stopped."

He said that putting the people of Gaza "in a continuous situation of confrontation and fighting" will only lead to more violence, saying "This is an excellent atmosphere for radicalism."

"But if you give them (Palestinians) hope, you open the borders, you let them go to school, let them look for good jobs, let them look for moderation. And we will succeed in allowing all those who want to have peace...to have the upper hand."

Pierre Krahenbuhl, head of the U.N. agency for Palestinian refugees, called the shelling a clear violation of international law. "These are premises that are protected, the sanctity of which has to be respected by all parties," he said Sunday on CBS' "Face the Nation."

About 1,800 Palestinians and 60 Israelies have been killed in Gaza nearly a month into Israel's forceful assault on Hamas, according to health officials.