Hours after the first phase of a polio vaccination campaign wrapped up in the central Gaza Strip, an Israeli airstrike hit the courtyard of a hospital there, underscoring the limited nature of the pauses in combat to allow health care workers to reach children.

The strike outside Al-Aqsa Martyrs Hospital occurred overnight, shortly before the World Health Organization said Thursday that the effort to vaccinate children had shifted to the southern part of Gaza, beginning the second phase of the campaign.

Israel has agreed to brief, staggered pauses in its military offensive in Gaza to allow health officials to make a frantic drive to vaccinate hundreds of thousands of children and avert a deadly polio outbreak.

The strike near the hospital, in the city of Deir al-Balah, 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 among some of the makeshift shelters on the hospital grounds that were being used by people who had fled their homes. Video taken by the Reuters news agency showed tents and shelters in ruins, their wooden beams flattened, and people’s belongings strewed outside the hospital, one of Gaza’s largest.

The Israeli military confirmed the strike but not the death toll nor the proximity to the hospital. It said it had struck a Hamas command center overnight to “remove an immediate threat,” which was “embedded” within a humanitarian area in Deir al-Balah.

“Numerous steps were taken to mitigate the risk of harming civilians, including the use of precise munitions” and aerial surveillance, the statement said, echoing words often employed by the military after airstrikes in Gaza.

Charitable group Doctors Without Borders said it was the fifth time since March that the hospital or its surroundings had been hit.

In the first phase of the vaccination campaign, the WHO inoculated more than 187,000 children over three days.

The second phase is expected to take place in southern Gaza over the next three days, before a third and final phase in northern Gaza. The effort aims to vaccinate about 640,000 children younger than 10 against the disease; the first polio case in Gaza in 25 years was recorded last month in a boy who is almost 1 year old.

The war in Gaza created the conditions for a resurgence of polio, said Juliette Touma, director of communications for UNRWA, the U.N. agency that helps Palestinians in Gaza. Displaced people are living in cramped tents with little access to clean water, she added.

“These are conditions unfit for humans,” she said.

The WHO, which is also a U.N. agency, said it had exceeded its target for the first phase by 30,000 children, as more than 2,180 workers fanned out across hospitals, temporary schools and camps for displaced people, visiting tents and areas destroyed by nearly 11 months of fighting.