Hours before families in Gaza lined up on Thursday to start the second phase of an emergency polio vaccination campaign, a deadly Israeli airstrike hit near a hospital in an area where a previous round of inoculations had just concluded.

Health officials have heralded the vaccination program, which began on Sunday and is built around a deal by Israel and Hamas for brief pauses in hostilities, as an unexpected success in the early going. The World Health Organization said the first phase, in the central Gaza Strip, finished on Wednesday and the second, in southern Gaza, began Thursday.

But the limited nature of those pauses was highlighted by the strike overnight, which killed four people and wounded a number of others, including women and children, according to Wafa, the Palestinian Authority’s official news agency. Witnesses said it landed in the courtyard of Al-Aqsa Martyrs Hospital in the central Gaza city of Deir al-Balah, where displaced civilians had taken shelter.

Photos and video taken by the Reuters news agency showed flattened tents and makeshift shelters at the site, with tarps, clothes and other belongings strewed on the ground outside the hospital, one of Gaza’s largest.

“We sought refuge in a safe place, in the courtyard of Al-Aqsa Hospital, displaced and sleeping peacefully, we found nothing but the airstrikes hitting us,” one woman, Iqbal Al-Zeidi, told Reuters.