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, Paralel.Az reports citing Reuters.

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

As in the first phase, humanitarian pauses in the fighting in Gaza between Israel and the Palestinian militant 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, 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.

At the same briefing, Peeperkorn said that three attempts by the U.N. health agency and partners to assist and evacuate patients from northern Gaza hospitals under evacuation orders have been thwarted this week.

"It's kind of unacceptable that we still struggle with what... should be routine humanitarian missions by now," he said, voicing deep concern about the affected patients. Fresh attempts will be made to reach them in the coming days, he added.