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

The first group of Israeli hostages have been released by Hamas in the Gaza Strip and handed over to the International Committee of the Red Cross, two sources briefed on the matter told ABC News.

The sources said the Red Cross has the hostages and that the "operation" to get them home has begun.

It was expected that 13 Israeli hostages would be freed by Hamas near Gaza's border with Egypt on Friday in a prisoner exchange deal. But it was unclear exactly how many were handed over to the Red Cross.

-ABC News' Matt Gutman and Morgan Winsor