The World Health Organization said on Tuesday it had been able to start its polio campaign in central Gaza and vaccinate tens of thousands of children despite Israeli strikes in the designated protected zone hours before.

As part of an agreement between the Israeli military and Palestinian militant group Hamas, humanitarian pauses in the year-long Gaza war had been due to begin early on Monday to reach hundreds of thousands of children.

However, hours before then, the U.N. humanitarian office said Israeli forces struck tents near al Aqsa hospital, inside in the zone, where it said four people were burned to death.

The U.N. Palestinian refugee agency UNRWA said one of its schools in the central Gazan city of Nuseirat, intended as a vaccination site, was hit overnight between Sunday and Monday, killing up to 22 people.

WHO spokesperson Tarik Jasarevic told a Geneva press briefing that over 92,000 children, or around half of the children targeted for polio vaccines in the central area, had been inoculated on Monday.

“What we have received from colleagues is that the vaccination went without a major issue yesterday, and we hope It will continue the same way,” he said.

Other humanitarian agencies have previously voiced concerns about the viability of the polio campaign in northern Gaza, where an Israeli offensive is under way.

Aid groups carried out an initial round of vaccinations last month, after a baby was partially paralyzed by the type 2 polio virus in August, in the first such case in the territory in 25 years.