The Israeli military pressed deeper into northern Gaza by ground on Friday after issuing a series of evacuation orders calling on Palestinians to flee, part of its escalating offensive against Hamas in the war-battered Gaza Strip.

The expansion of ground operations came after the Palestinian health authorities said on Thursday that dozens of people, including children, were killed in Israeli strikes on a school turned shelter in the Tuffah neighborhood of Gaza City. On Friday, the Israeli military said the strikes were targeting well-known militants in a Hamas command and control center, without naming them.

The evacuation orders have brought renewed hardship to Palestinians who had already endured displacement from their homes and miserable conditions during the first 15 months of the war. A shaky two-month cease-fire between Israel and Hamas collapsed in March after the two sides failed to reach an agreement to extend it, ending a brief respite for Palestinians in Gaza.

The Israeli military has since embarked on a major bombing campaign and seized territory in Gaza in a tactic that Israeli officials have said was intended to compel Hamas to release more hostages.

As the Israeli military operation expanded, Hamas’s military wing on Friday appeared to threaten the remaining Israeli and foreign hostages held in Gaza. In a statement that did not mention the plight of its own people, Hamas said its fighters were holding some captives in the evacuation zones under “strict security measures that are extremely dangerous to their lives.”

The armed group has in the past threatened the well-being of hostages in the face of Israeli bombardments.