Thousands of children in Gaza City were receiving a second dose of polio vaccine this weekend in an effort that was delayed by intense Israeli bombardment and mass evacuation orders in northern Gaza, the United Nations and other aid agencies said.

The second phase of the vaccination campaign was originally set to begin on Oct. 23 across the north of the territory, but it was postponed due to a lack of assurances about pauses in the fighting and bombardment to ensure the safety of health workers, the World Health Organization and UNICEF said in a statement on Friday.

The first round of vaccinations in September took place across northern Gaza. Since then, the Israeli military has launched an intense offensive in northern Gaza against what it has said is a resurgence of Hamas in the area.

A humanitarian pause for the second phase of the vaccination campaign was only assured for Gaza City, according to the U.N. agencies. They said that around 15,000 children under 10 in northern towns where the Israeli military has been carrying out the offensive over the last few weeks “remain inaccessible and will be missed during the campaign, compromising its effectiveness.”

COGAT, the Israeli government agency that oversees policy in Gaza and the occupied West Bank, said on Sunday that 58,604 children under 10 had been vaccinated in northern Gaza since the second phase of the campaign began a day earlier. It added that Israel would continue to work to “facilitate an effective vaccination campaign.”

The Gazan Health Ministry confirmed the number of vaccinations, and the campaign was expected to continue through Monday.