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

Hamas announced that it will be delaying the release of Israeli hostages, accusing Israel of violating the cease-fire agreement regarding aid trucks and the agreed-upon terms for the release of Palestinians imprisoned in Israel.

The Al-Qassam Brigades -- the armed wing of Hamas -- said it had decided to delay Saturday's scheduled second round of hostage releases until Israel committed to allowing aid trucks to enter northern Gaza. Hamas was expected to release 13 Israeli hostages in exchange for the release of 39 Palestinians imprisoned in Israel on Saturday, sources told ABC News.

"The Al-Qassam Brigades decides to delay the release of the second batch of hostages until the occupation adheres to the terms of the agreement regarding the entry of relief trucks into the northern Gaza Strip, and due to its failure to adhere to the agreed-upon criteria for releasing prisoners," Al-Qassam Brigades said in a statement.

Hamas leader Osama Hamdan, who is based in Lebanon, said relief trucks that arrived in northern Gaza today were half of the number that was agreed upon.

"There are violations committed by the Israelis in implementing the terms of the truce, some of which occurred yesterday and were repeated today," Hamdan said Saturday, adding that another issue at stake was the reported shooting of two Palestinians on Friday as they tried to reach northern Gaza.

A senior Israeli political source told ABC News that Israel did not violate the agreement. The IDF said about 200 trucks carrying humanitarian aid are expected to enter Gaza today, after 200 trucks with aid entered Gaza yesterday via the Rafah crossing and delivered their cargo to international aid organizations.

The Palestinian Red Crescent said 61 trucks carrying food, water and medical supplies headed to northern Gaza on Saturday, the largest aid convoy to reach the area since the start of the war.

-ABC News' Ayat Al-Tawy, Nasser Atta, Jordana Miller