(Adds details about source of artillery shell)
By Mark Felsenthal
WASHINGTON, July 30 (Reuters) - The White House condemned the shelling on Wednesday of a United Nations school in Gaza that local authorities estimated killed at least 15 Palestinians sheltering there.
"We are extremely concerned that thousands of internally displaced Palestinians who have been called on by the Israeli military to evacuate their homes are not safe in U.N. designated shelters in Gaza," White House National Security Council spokeswoman Bernadette Meehan said. "We also condemn those responsible for hiding weapons in United Nations facilities in Gaza."
The head of the United Nations Relief and Works Agency, which runs the school, said the organization's initial assessment was that Israeli artillery had hit the building. However, the United States, in its statement condemning the shelling, did not assign responsibility for the attack.
Some 3,300 Palestinians, including many women and children, were taking refuge in the school in Jabalya refugee camp when it came under fire around dawn, UNRWA said.
An Israeli military spokeswoman said militants had fired mortar bombs from the vicinity of the school and troops shot back in response.
UNRWA said on Tuesday it had found a cache of rockets concealed at another Gaza school - the third such discovery since the conflict began. It condemned unnamed militant groups for putting civilians at risk. (Reporting By Steve Holland and Mark Felsenthal; Editing by Doina Chiacu and Sandra Maler)