At least 151 patients at the Al-Shifa Hospital in northern Gaza, which has been under siege for days, have been transferred to other medical facilities, the World Health Organization said Friday.

The mission to transfer the patients occurred on Wednesday, according to a statement from WHO.

It was the third mission undertaken in less than a week to move patients, including 31 infants, according to WHO.

The Israel Defense Forces, which raided the hospital last week, claimed Hamas was using the hospital to conceal its military operations and to hold hostages.

The Palestine Red Crescent Society joined WHO in carrying out the transfer of patients, according to the WHO statement.

Most of the patients were transferred to the European Gaza Hospital, according to WHO.

The patients moved in the latest mission included 73 severely ill patients, 18 of whom are dialysis patients, 26 with serious spinal injuries and eight with severe chronic conditions, according to WHO.

The evacuation was described by WHO as a "high-risk mission," saying it occurred over a 20-hour period as intense fighting and shelling continued in proximity of the hospital.

About 100 patients remain at the hospital, according to WHO.

-ABC News' Eric Strauss

President Joe Biden said Friday that the release of 24 hostages held by Hamas in Gaza is "just the beginning" of a plan to free 50 of the more than 230 captives over four days.

“It's only a start, but so far it's gone well,” Biden said in a televised address from Nantucket, Massachusetts, where he and first lady Jill Biden are vacationing.

He said Friday's hostage release is part of a deal "reached by extensive U.S. diplomacy, including numerous calls I've made from the Oval Office to leaders across the region," including calls with Israeli Prime Minister Benjamin Netanyahu, Egyptian President Abdel Fattah El-Sisi and the emir of Qatar.

“All of these hostages have been through a terrible ordeal, and this is the beginning of a long journey of healing for them,” Biden said. “Today has been a product of a lot of hard work and weeks of personal engagement.”

Biden said that under the deal, more hostages would be released over the next three days with 50 being "our goal." But he added, "We also will not stop until we get these hostages brought home and an answer to their whereabouts.”

The president said U.S. officials do not know when Americans -- including two women and 4-year-old Abigail Edan, whose parents were killed by Hamas terrorists -- will be among those that are released or their conditions. He added, “We expect it to occur” and that it’s his “hope and expectation” that it “will be soon.”

Biden said the four-day cease-fire that was negotiated for the release of the hostages will allow time to "accelerate and expand humanitarian assistance going into Gaza."

Speaking of the children freed on Friday, one just 2 years old, Biden said, “The teddy bears waiting to greet those children at the hospital are a stark reminder of the trauma these children have been through at such a very young age.”

-ABC News' Justin Gomez

There were 200 trucks carrying aid that entered Gaza through the Rafah border crossing on Friday -- the first day of the humanitarian pause.

Of those 200 trucks, 137 unloaded goods at the reception point for the United Nations Relief and Works Agency for Palestine Refugees in the Near East, the aid organization said. It was the largest single-day delivery of aid since Oct. 7, the agency said.

Also, 129,000 liters of fuel and four trucks of gas crossed into Gaza through the border, the U.N. said. President Joe Biden said Friday both fuel and cooking gas were delivered to Gaza.

Twenty-one critically injured patients from northern Gaza were also evacuated through the border.

-ABC News' Will Gretsky

Twenty-two of the 24 Israeli and foreign citizens released by Hamas on Friday were first taken to the Hatzerim Air Force base in Israel before being transferred by military helicopters to hospitals, where they are to be reunited with their families, according to the Israel Defense Forces.

Two Israeli citizens were taken directly to hospitals by ambulance, the IDF said.

The returning hostages were escorted by IDF personnel, including medical personnel, the IDF said. IDF representatives were with the family members of the hostages to give them regular updates on the release of their loved ones, the IDF said.

Israeli Air Force helicopters were waiting at Hatzerim base to fly the returnees to the hospitals, according to the IDF.

-ABC News' Matt Gutman

The "key" to securing the hostage deal between Israel and Hamas "was bearing down on both sides" to "get specific" on the details, according to U.S. National Security Advisor Jake Sullivan.

The agreement that was reached on Wednesday was mediated by the United States, Qatar and Egypt.

"The big move happened late last week when [U.S.] President [Joe] Biden spoke first with [Israeli] Prime Minister [Benjamin] Netanyahu and then with the emir of Qatar to say, how many hostages, for how any days in return for how many Palestinian prisoners," Sullivan told ABC News' George Stephanopoulos in an interview Wednesday on "Good Morning America."

"And it was really the work to refine those details that finally produced the breakthrough that we now see resulting in the announcement of a pause of hostilities and the first return of prisoners, of hostages," he added.

-ABC News' Morgan Winsor