Israel seized and closed the Rafah border crossing between Gaza and Egypt on May 7, disrupting a vital route for people and aid into and out of enclave.
"At a time when the people of Gaza are facing starvation, we urge Israel to lift the blockade and let aid through," WHO Director-General Tedros Adhanom Ghebreyesus told a news conference in Geneva, describing the situation in the Palestinian enclave as "beyond catastrophic".
"Without more aid flowing into Gaza we cannot sustain our lifesaving support of hospitals and populations," he said.
Israel says U.N. agencies are to blame for not distributing aid more efficiently within the enclave, creating backlogs of supplies.
Tedros said Israel's move had impacted six hospitals and nine primary health centres and caused 70 shelters to lose their medical facilities.
"Daily consultations have fallen by close to 40% and immunization by 50%," he said. "Approximately 700 seriously ill patients who would have otherwise been evacuated for medical care elsewhere are stuck in a war zone."
Gaza's healthcare system has essentially collapsed since Israel began its military offensive there after the Oct. 7 cross-border attack on Israel by Palestinian Hamas.
Tedros said that Gaza's Al-Awda Hospital in northern Gaza remained under siege since Sunday, with 148 hospital staff and 22 patients and the people accompanying them trapped inside. He said that fighting near Kamal Adwan Hospital, also in northern Gaza, had jeopardised its ability to care for patients.
"These are the only two functional hospitals remaining in northern Gaza," Tedros said. "Ensuring their ability to deliver health services is imperative."
