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

Israel's Cabinet has approved a deal that would allow for the release of at least 50 hostages over the course of a four-day pause in hostilities, Israel's Prime Minister said early Wednesday morning. As a part of this deal, 150 Palestinian prisoners will also be released by Israel.

Prime Minister Benjamin Netanyahu said in a release the “Israeli government has approved the outline to the first phase” of bringing hostages home, with at least 50 being released over the course of the four-day pause in fighting.

The Government of Israel, the IDF and the Israeli security forces will continue to fight the war for the return of all hostages, the elimination of Hamas and to ensure that Gaza will not pose a threat to Israel, according to Netanyahu.

Earlier, sources told ABC News 42 hostages would be released.

A short while later, Hamas released its own statement early Wednesday morning local time, which had some of the same terms announced by Prime Minister Benjamin Netanyahu.

Hamas confirmed via Telegram a temporary cease-fire of four days, with the release of 50 women and children who’ve been held hostage.

In exchange, Hamas said Israel agreed to release 150 Palestinian women and children. Hamas said the juveniles released would be under 19 years old.

President Joe Biden praised the release of hostages Sunday from the Gaza Strip and said he hopes the four-day cease-fire will be extended.

"Look, you know the deal calls for every 10 hostages released, we extend another day. So, I'm hopeful this is not the end, it's going to continue. But we don't know. But I get a sense that all the players in the region, even the neighbors who haven't been directly involved [up] to now, are looking for a way to end this so the hostages are all released and Hamas is completely, how can I say it, no longer in control of any portion of Gaza," Biden said at a press conference.

"I'd like to see the pause go on as long as prisoners keep coming out," he added.

Biden said there are probably other militant groups that have hostages in the Gaza Strip, and that the condition of the other Americans being held in Gaza is unclear.

The president's remarks come as 4-year-old Abigail Idan was among the hostages released Sunday.

Idan's parents were killed in front of her during the October attack in southern Israel, U.S. officials have said.

"She's been through a terrible trauma," Biden said. "What she endured is unthinkable."

Israel and Hamas are currently on the third day of a four-day pause in fighting during which Israel plans to release 150 Palestinians and Hamas will release 50 of the over 200 people it is thought to have taken hostage on Oct. 7.