United Nations officials voiced concerns on Friday that an Israeli offensive and evacuation orders in northern Gaza might affect the second phase of its polio vaccination campaign set to start next week.

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, Reuters reported. 

As in the first phase, humanitarian pauses in the fighting in Gaza between Israel and the Palestinian resistance group Hamas are planned in order to reach hundreds of thousands of children.

In Gaza's north, the Israeli military has been pursuing an offensive in recent days, sending its troops into Jabalia, the largest of Gaza's eight historic refugee camps, and the nearby towns of Beit Hanoun and Beit Lahiya.

"I am of course, concerned about the developments in the north, and specifically with these evacuation orders," Rik Peeperkorn, the WHO representative for the occupied Palestinian territory, told reporters in Geneva, saying dozens of healthcare facilities across the Strip were under such orders by the Israeli military.

Jean Gough, a UNICEF special representative also voiced concern and described conditions as "more complicated" than in the first phase of the vaccination campaign last month. The first vaccinations are set to start in central Gaza on Monday, before moving to the south and then the north, she added.