Geneva, Switzerland - The World Health Organization said four children were among the six people wounded Saturday in a strike on a polio vaccination center in northern Gaza.

"We have received an extremely concerning report that the Sheikh Radwan primary health care center in northern Gaza was struck today while parents were bringing their children to the life-saving polio vaccination in an area where a humanitarian pause was agreed to allow vaccination to proceed," said WHO chief Tedros Adhanom Ghebreyesus.

"Six people, including four children, were injured."

Tedros did not specify who carried out the strike but a source in Gaza's civil defense agency told AFP that it was "an Israeli quadcopter that fired two missiles which hit the wall of Sheikh Radwan clinic."

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The Israeli military did not immediately comment.

The military has been pounding northern Gaza for weeks in a major offensive it says is aimed at stopping Hamas militants from regrouping.

UN agency chiefs have spoken of an "apocalyptic" situation in north Gaza as the area has been "denied basic aid and life-saving supplies".

"A WHO team was at the site just before" Saturday's strike, Tedros said.

"This attack, during humanitarian pause, jeopardizes the sanctity of health protection for children and may deter parents from bringing their children for vaccination," he added.

The WHO launched the second round of child polio vaccinations in northern Gaza on Saturday after Israeli bombing halted an earlier attempt.

After the besieged Palestinian territory confirmed its first case of polio in 25 years, the vaccination drive began on September 1 with a successful first round.

According to the WHO, some 119,000 children in the north are awaiting their second dose, while 452,000 have been vaccinated in central and southern Gaza.

Typically spread through sewage and contaminated water, poliovirus is highly infectious. It can cause deformities and paralysis, and is potentially fatal, mainly affecting children under the age of five.