GAZA CITY, Gaza Strip (AP) — Health officials in the Gaza Strip say 25 Palestinians have been killed since Israel began its ground operation in Gaza late yesterday, including three teenage siblings who died when a tank shell hit their home.

The father of the teens says they were sleeping when the shell struck, and that he had to dig them out from under the rubble.

Israel says it is going to great lengths to avoid civilian casualties. It blames them on Hamas, accusing the militant group of firing from within residential neighborhoods and using civilians as "human shields."

The Israelis pushed deeper into Gaza today, firing volleys of tank shells and clashing with Palestinian fighters. Israel says it's trying to stop rocket attacks and destroy the network of Hamas tunnels into Israel. But some hard-liners in Israel want their military to completely crush Hamas and drive it from power in Gaza.

The streets of Gaza City have been largely deserted today. The sound of steady shelling could be heard across Gaza as Israel continued to strike targets from the air. Buildings shook as missiles hit.