Israel’s military intensified attacks on the southern Gaza Strip on Saturday, opening what could be the next phase of its war against Hamas militants, as negotiators pushed to reinstate a cease-fire that unraveled after a weeklong pause in the fighting.

Israel’s air force said Saturday it carried out an “extensive attack” against militants overnight in Khan Younis, a city in southern Gaza crowded with displaced people who fled prior weeks of Israeli bombing. The air force said it carried out 400 strikes over the past day. Palestinians in the area said explosions shook buildings throughout the night, while the director of a large hospital in the city said the emergency room was flooded with casualties.