Nearly half of the health care centers and dispensaries in conflict-affected areas in Lebanon are now closed, jeopardizing access to health care, according to the World Health Organization

"Increasing conflict, intense bombardment and insecurity are forcing a growing number of health facilities to shut down, particularly in the south," the WHO said in a statement Tuesday. "Hospitals have had to close or evacuate due to structural damage or their proximity to areas of intense bombardment."

The World Health Organization said it has verified 23 attacks on health care in Lebanon, killing 72 and injuring 43 health workers and patients since the escalation of hostilities on Sept. 17.

Fifteen incidents impacted health facilities and 14 impacted health transport, according to WHO.

"Hospitals in Lebanon are already under massive strain as they strive to sustain essential health services while dealing with an unprecedented influx of injured people. Understaffed and under-resourced, the health system has been struggling to maintain uninterrupted services to all those in need with supplies being depleted and health workers exhausted," WHO said.

-ABC News' Guy Davies

Escalating violence in northern Gaza is having "a disastrous impact on food security for thousands of Palestinian families," the United Nations World Food Programme warned on Tuesday as aid groups issue sharp warnings.

“The north is basically cut off and we’re not able to operate there,” Antoine Renard, WFP country director for Palestine, said in the release. “WFP has been on the ground since the onset of the crisis. We are committed to delivering life-saving food every day despite the mounting challenges, but without safe and sustained access, it is virtually impossible to reach the people in need."

The health systems in northern Gaza have "all but collapsed," United Nations Relief and Works Agency for Palestine Refugees in the Near East Commissioner-General Philippe Lazzarini said.

Lazzarini said they are "unable to reach" UNRWA teams in northern Gaza "due to telecommunications cuts."

The Israel Defense Forces said they are assisting patients, personnel and hospital staff from Kamal Adwan Hospital in northern Gaza "to other functioning hospitals in Gaza," in a statement Tuesday. An Israeli agency that manages logistics inside of Gaza, including the flow of aid into Gaza, is leading the transfer of patients and staff, the IDF said.

Three hospitals in northern Gaza are inside of the zone where Israeli forces have asked people to evacuate.

The IDF also acknowledged they have been operating "in the Jabalia area" in northern Gaza for "over a week," in a statement Tuesday. The IDF claims they conducted "targeted raids on dozens of terrorist infrastructure sites in the area, eliminated dozens of terrorists, and confiscated numerous weapons," in the Jabaliya area during operations there, the statement said.

-ABC News' Sami Zyara, Diaa Ostaz and Jordana Miller

Over 92,800 children in Gaza were vaccinated on Monday, the first day of the second phase of the polio vaccine campaign, the United Nations Children's Fund said Tuesday.

"Despite the incredibly complex situation in Gaza, the second phase of Gaza's polio vaccination campaign began smoothly yesterday, reaching over 92,800 children with polio vaccines and administering Vitamin A to more than 76,000 children between the ages of 2 and 10," UNICEF said in a statement Tuesday.

"This campaign is crucial not only for preventing the resurgence of polio but also for safeguarding the long-term health of Gaza's children, who are already facing huge vulnerability due to ongoing conflict, restricted access to healthcare, and malnutrition. Each dose of the vaccine is a lifeline, in an environment where every safeguard counts," UNICEF said.

U.S. officials sent a letter to Israeli officials demanding that Israel take steps within 30 days to improve the humanitarian situation in Gaza, or Israel will face consequences with a potential change in U.S. policy, two Israeli sources confirmed to ABC News.

U.S. Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin sent a letter to Israeli Minister of Strategic Affairs Ron Dermer and Israeli Minister of Defense Yoav Gallant Monday focusing on increasing the supply of humanitarian aid into Gaza by the beginning of winter, facilitating the aid delivery route through Jordan and ending the "isolation" of northern Gaza.

"Failure to demonstrate a sustained commitment to implementing and maintaining these measures may have implications for US policy under NSM-20 and relevant US law," the letter stated.

The National Security Memorandum, or NSM-20, states the secretaries of State and Defense are "responsible for ensuring that all transfers of defense articles and defense services" by the departments under "any security cooperation or security assistance authorities are conducted in a manner consistent with all applicable international and domestic law and policy, including international humanitarian law and international human rights law," according to the law.

Deputy Pentagon press secretary Sabrina Singh confirmed Austin and Blinken sent a letter to their Israeli counterparts over humanitarian conditions in Gaza, but refused to give additional details.

"I can confirm that Secretary Austin with Secretary Blinken, they co-signed a letter that went to their Israeli counterparts. This was personal, private correspondence, so I'm not going to get into more specifics of it, other than it was expressing concern about the humanitarian conditions in Gaza," Singh said on Tuesday.

The letter was first reported by Israeli media and Axios reporter Barak Ravid.

-ABC News' Jordana Miller, Dorit Long and Matt Seyler