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UNITED NATIONS, Jan 6 (Reuters) - A U.N. official in Gaza said a school where dozens of Palestinians were killed by tank shells on Tuesday was clearly marked with a U.N. flag and its location had been reported to Israeli authorities.

John Ging, director of operations in Gaza for UNRWA, the United Nations Relief and Works Agency, said the death toll in the Israeli artillery strike near the school in Jabalya refugee camp was 30 dead with another 55 people injured.

Medical officials on the spot have said more than 40 people were killed.

Ging told reporters at U.N. headquarters by videolink from Gaza that three artillery shells landed at the perimeter of the school where 350 people were taking shelter.

He said UNRWA regularly provided the Israeli army with exact geographical coordinates of its facilities and the school was in a built-up area. "Of course it was entirely inevitable if artillery shells landed in that area there would be a high number of casualties," he said.

Casualty numbers were still being assessed but the latest figures were 30 dead and 55 injured, including at least five critically, Ging said.

The Israeli military said it is looking into the incident at al-Fakhora school in Jabalya on the fourth day of a ground assault launched after a week of air strikes failed to end Hamas rocket salvos against Israeli towns.

Asked whether Hamas militants were in the area of the Jabalya school at the time of the strike, Ging said it was the scene of clashes "so there's an intense military and militant activity in that area."

He said U.N. staff vetted Palestinians seeking shelter at their facilities to make sure militants were not taking advantage of them. "So far we've not had violations by militants of our facilities," he said.

Ging also said three Palestinians were killed in a separate Israeli air strike near another U.N. school in an area where there was no fighting at the time.

On Tuesday morning a building next to a U.N. health center was hit by Israeli fire and 10 people were injured, including seven staff and three patients at the health center, he said.

Another shell landed near a third U.N. school on Tuesday but it was empty, Ging said.

He called for an independent investigation of the strikes near U.N. facilities. (Editing by Alan Elsner)