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

The United States is "very hopeful" in the hostage deal between Israel and Hamas but "cannot be fully confident" until the abductees return home safely, according to White House National Security Advisor Jake Sullivan.

"You can never have full confidence until you actually see hostages back in the arms of their families," Sullivan told ABC News' George Stephanopoulos in an interview Wednesday on "Good Morning America."

"But I am very hopeful that there will be a pause, these hostages will begin to be released," he continued, "and then I am hopeful that we can build on the release of this initial 50 to get more hostages home so that every last person currently being held by the terrorist group Hamas gets home safely to their family."

The whereabouts of nine U.S. citizens and one U.S. permanent resident, or green card-holder, remains unknown in the wake of Hamas' Oct. 7 attack on Israel, according to Sullivan, who said it's unclear whether they were among the hundreds of people who were taken hostage and brought back to the neighboring Gaza Strip.

"We don't know for certain that all 10 are still alive and we’ve been honest about that -- what we know and what we don't know,” he added. "We know that among those 10, three are women and children -- two women one child. And we believe that those three will be among the 50 who are released."

But Sullivan cautioned: "Until we actually see them come out, be in the arms of their loved ones, get home safely, we cannot be fully confident. So we are going to wait as the families are waiting with bated breath until they come but and then until every last American comes out."

-ABC News' Morgan Winsor

President Joe Biden released a statement Tuesday night thanking Sheikh Tamim bin Hamad Al-Thani of Qatar and President Abdel-Fattah El-Sisi of Egypt for their "critical leadership and partnership" in helping to broker a deal between Hamas and Israel over the release of hostages.

Biden also thanked Israeli Prime Minister Benjamin Netanyahu for his commitment to the temporary pause ensuring the deal can be carried out and "bring home additional American hostages." Biden said he "will not stop until they are all released."

Senate Majority Leader Chuck Schumer said he was "pleased and relieved" over being "close" to seeing the release of the first 50 out of 240 hostages.

Schumer said he will continue working for the release of the remaining hostages, and called it a "positive development" that the pause in fighting will allow aid to flow to Palestinians in Gaza.

Biden said as president he has "no higher priority than ensuring the safety of Americans held hostage around the world."

Three Americans are expected to be part of the initial group of hostages that will be released as part of a deal negotiated between Hamas, Israel, Qatar and the United States, according to a senior administration official.

They include 3-year-old Abigail Mor Idan, who was orphaned by the Oct. 7 attack and whose birthday is on Friday, the official said.

The deal will see the release of 50 hostages -- women and children -- over the course of several days.

Officials would not get into specifics about the number of additional Americans believed to be held hostage -- only reiterating that there are 10 unaccounted-for Americans.

-ABC News' Molly Nagle and Justin Gomez

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