The health care system in Gaza City is coming under fire and being pushed toward collapse as Israeli troops bear down

Thousands of Palestinians have been forced to flee Gaza City since Israel launched its ground offensive last week.

Israeli tanks have moved into multiple neighborhoods of Gaza City over the past 24 hours, with journalists working for CNN in the city reporting heavy shelling, explosions and the sound of gunfire.

Israel's military says troops have "begun pushing into the heart of Gaza City," renewing its call for thousands of war-weary civilians to evacuate.

An estimated 1 million Palestinians were living in the Gaza City region before warnings to evacuate began ahead of the offensive. The Israeli military estimates 350,000 people have left the city. The U.N. estimates that more than 238,000 Palestinians have fled northern Gaza over the past month. Hundreds of thousands more have stayed behind.

A Palestinian medical charity said Tuesday that Israel destroyed its main center in Gaza City after ordering its evacuation. The Palestinian Medical Relief Society said an Israeli strike reduced its six-story building in the central Samer area to rubble.