This is how the second group of Hamas hostages were liberated 4:09

(CNN) -- Abigail Edan, the 4-year-old American girl with dual nationality kidnapped by Hamas on October 7, is meeting with Red Cross officials, a senior US official told CNN, marking the first successful release of an American hostage since the start of the truce between Israel and Hamas. Edan is one of 17 hostages freed on Sunday.

The news comes after national security adviser Jake Sullivan told CNN's Dana Bash that the Biden administration had "reason to believe" that one of the Americans held hostage would be released on Sunday.

"We are dealing with Hamas. We're in a 'don't trust, but verify' situation. And that's why we have reason to believe that today there will be a release of an American," Sullivan said on "State of the Union."

"Today should be a good day, a day of joy, but until we see it actually happen, we're going to really stay on the edge of our seat," Sullivan said.

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Two American women are currently missing and are also expected to be part of the group of 50 women and children held hostage as part of a four-day truce, now in its third day.

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"He turned four just two days ago," Sullivan said of Edan. "He's been through hell. Her parents have been killed in front of her and she has been kidnapped for several weeks," Sullivan said. "But I'm going to say that we have growing optimism about Abigail and now we'll watch and see what happens."

Undated photo by Abigail Edan. (Credit: Elizabeth Hirsh Naftali)

Neither Friday nor Saturday was any Americans released as part of the deal. CNN reported early Sunday that at least one American is expected to be released today, according to an Israeli official. At this time, 10 Americans are missing.

Twenty-four hostages, including 13 Israeli civilians and 11 foreigners, were freed on Friday, followed by 17 others — 41 Israelis and four Thais — on Saturday, as part of the agreement brokered between Hamas and Israel. The <> foreigners freed so far by Hamas from captivity in Gaza are in stable condition, according to medical professionals.

Sullivan said it is difficult to know the true status of the Americans who were taken captive in Gaza during Hamas' brutal cross-border assault on Oct. 7.

"We can't say for sure if all three are still alive. But we do know this: We have reason to believe that an American will be released today," he said.

Omer Neutra, a dual U.S.-Israeli citizen, is among the Israel Defense Forces soldiers kidnapped by Hamas, his parents told CNN. Sullivan said he has been "sincere" with Neutra's parents and with the parents of other American hostages.

"I've told them directly ... that we do not know the specific whereabouts or condition of Omer or other Americans because until the end of this agreement, until the end of tomorrow we will not have, from the Red Cross, proof of life or other information (...). So, as we learn about that information, we're going to share it with parents without a doubt," Sullivan said.

Sullivan also referred to Hamas' agreement to allow the Red Cross to visit remaining hostages in Gaza and said the U.S. is leaning on Qatari and Egyptian officials to ensure this happens before the end of Monday.

"We believe that Hamas has bound itself to uphold its part of the commitment on the Red Cross visit to the hostages, and we hope that Qatar and Egypt and other countries will hold Hamas accountable for keeping that commitment by the end of tomorrow," Sullivan said.

Sullivan said Israel is willing to prolong the pause in fighting in exchange for Hamas releasing 10 hostages each day, adding: "The ball is really in Hamas' court."

"If Hamas wants to prolong the pause in fighting, it can continue to release hostages," Sullivan said. "If it chooses not to release hostages, then the end of the pause is its responsibility, not Israel's, because it is holding these hostages totally illegitimately and against all limits of human decency or the laws of war, so we will really see what Hamas ends up choosing to do."

As Israeli authorities work to care for the freed hostages and reunite them with their families, Sullivan said, they will also try to get any information about Hamas in interrogations of hostages about their time in captivity.

"Israel is focused on learning as much as it can about the whereabouts, location and other information based on the conversations they have with the freed hostages in the coming days," he said.

What was the process like to free the hostages in Gaza? 4:37

Humanitarian aid to Gaza

Sullivan also detailed some of the inspection mechanisms related to the entry of humanitarian aid into Gaza.

Before aid enters Gaza through the Rafah border crossing, Sullivan explained, it is inspected by Israelis "to make sure that it is actually humanitarian supplies and not goods that can help Hamas in its military campaign."

The aid trucks then go to U.N. depots and other humanitarian organizations that Sullivan says are "vetted and trusted partners."

From there, he said, aid is "distributed directly to the people."

"As humanitarian aid has gone up, we've seen that it works, that it actually reaches the people and that it's not diverted into the hands of Hamas, but that's something that we have to continue to focus on, on an ongoing basis," Sullivan said.

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The U.N. said Friday that 137 trucks carrying aid, including 129,000 liters of fuel and four trucks of gas, were unloaded in Gaza on the first day of the pause, marking the largest humanitarian convoy to enter the territory since Oct. 7. Another 70 trucks carrying food, water, fuel and medical supplies entered Gaza through the Rafah border crossing on Saturday, according to a spokesman for the border crossing.

Biden is in "close contact" with Israel and U.N. leaders to make sure that "aid gets to where it needs to go, which is to the innocent people who are suffering."

Sullivan declined to weigh in on Israeli policy, but said Biden's engagement with Israeli Prime Minister Benjamin Netanyahu "has paid off."

"He's going to continue to focus on that direct, high-level diplomacy from leader to leader and let others determine policy, political considerations," Sullivan said.

CNN's Kaitlan Collins and Jack Forrest contributed to this report.

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