U.S. National Security Council spokesperson John Kirby said Monday that an estimated seven to 10 American hostages remain in the war-torn Gaza Strip.
"We know that there are other Americans being held hostage. We're certainly going to do everything we can hour by hour to get them released," Kirby told ABC News' George Stephanopoulos in an interview on "Good Morning America."
"We know that there were 10 unaccounted for, missing Americans," he added. "We think that the large majority of them are in this hostage population. But we don't have great fidelity past that."
John Kirby talks possible release of more American hostages in Gaza cease-fire
The National Security Council spokesperson discusses where things stand as far as more hostages being released before the cease-fire runs out.
Kirby noted that it’s also "difficult to know" exactly how many total hostages are in Gaza and which groups are currently holding them.
"We have to assume there are a couple hundred or so," he said. "We also have to assume, George, that Hamas isn't holding all of them -- that the other groups, potentially the Palestinian Islamic Jihad, for instance, another radical extremist terrorist group there operating out of Gaza, could be holding some. But it's going to be up to Hamas now to track all them down and see if we can arrange for their exchange."
With a four-day cease-fire between Gaza's militant rulers, Hamas, and Israel set to expire Tuesday morning, a fourth and final prisoner swap is expected to take place Monday between 4 and 6 p.m. local time, according to Israeli authorities.
"We're into day four now and so our hope and expectation is yet another instalment of hostages will get released sometime later today," Kirby told ABC News. "There have been delays in the past couple of days. We'll see if we have to work through any delays today."
"We would very much like to see this pause continue and more hostages exchanged as a result," he continued. "Isreal has said they're willing to do that. It's really going to be up to Hamas now to determine whether they're willing to come up with more hostages in coming days."
-ABC News' Morgan Winsor