On Thursday, the agency said that 12 of its employees were killed in Gaza.
"We, as UNRWA, have already lost 14 staff members. They were teachers, engineers, guards and psychologists, an engineer and a gynecologist," Lazzarini said in a statement, adding that most of the organization's 13,000 employees in the Gaza Strip were forced to leave their homes.
He also added that at least "400,000 displaced people are now in UNRWA schools and buildings," and the situation is approaching a humanitarian catastrophe.
"Imposing a siege and bombarding civilian infrastructure in a densely populated area will not bring peace and security to the region," Lazzarini said.
On October 7, Palestinian group Hamas launched a surprise large-scale rocket attack against Israel from the Gaza Strip, prompting Israel to declare a state of war the following day and launch retaliatory strikes. Both Israel and Palestine have reported over 1,000 deaths and thousands of injured as a result of the escalation.