Israel pounded Gaza City and its outskirts overnight, residents said Thursday, as the military announced the first steps in its push to the enclave's largest population center.

The newly approved plan authorizes the call-up of roughly 60,000 reservists, deepening fears the campaign will worsen the already catastrophic humanitarian crisis in the Gaza Strip.

"We are not waiting. We have begun the preliminary actions, and already now, IDF (army) troops are holding the outskirts of Gaza City," said the Israeli military.

Israel's plans to expand the fighting and seize Gaza City have sparked international outcry as well as domestic opposition. The Red Cross became the latest voice to condemn the plan on Thursday, calling it "intolerable."

Ahead of the offensive, the Israeli military said the call-up of the reservists would begin in early September, adding the second phase of the so-called operation "Gideon's Chariots" had begun.