Seventeen Americans are missing, including an unknown number of Americans being held hostage by Hamas, National Security Council spokesman John Kirby said.

"I think we all need to steel ourselves for the very distinct possibility that these numbers will keep increasing and that we may, in fact, find out that more Americans are part of the hostage pool," Kirby said.

He said the conditions and whereabouts of the hostages are not known.

"We don't know where they are, we don’t know if they're all in one group, or broken up into several groups. We don't know if they're being moved, and with what frequency and to what locations. All of those questions we're working hard to answer," he said.

Pressed by ABC News if U.S. officials have seen any proof of life, Kirby said, "I am not aware of any specific proof of live on any individual hostage."

Asked if the U.S. has had any communication with Hamas, directly or through allies, Kirby said, "We're in discussions not only with the Israelis about what hostage recovery can look like, but with other allies and partners in the region. And there are some countries like Qatar that have open lines of communication with Hamas. So, of course, we're casting the net wide."

"We haven't made any policy options or operational decisions with respect to hostage recovery at this time," he said.

Kirby said there is no "specific evidence that Iran was directly involved with these specific sets of attacks" on Israel, but he added, "We are going to keep looking at it -- the book is not closed."

In recent days, the White House has consistently said there was no evidence pointing to Iran’s participation but said Iran was broadly complicit with their past funding and support for Hamas.

-ABC News' Mary Bruce

Rep. Cory Mills, R-Fla., a freshman lawmaker and veteran who served in Iraq, has traveled to Israel and is working to help evacuate Americans through Jordan, a spokesperson confirmed to ABC News. News of his trip was first reported by The Floridian.

-ABC News' Ben Siegel and Lauren Peller

Nine workers from the United Nations Relief and Works Agency have been killed from strikes on Gaza, the agency said.

"UNRWA staff are working around the clock to respond to the needs of the displaced in the shelters," the agency said. "However, some are overcrowded and have limited availability of food, other basic items and potable water."

Reports of an infiltration into Israeli air space from Lebanon have been determined to be a false alarm, the Israel Defense Forces said.

Multiple alerts were heard along the northern border.

Israel and Hezbollah were openly exchanging fire at the northern border earlier in the day.

In remarks with Israeli Prime Minister Benjamin Netanyahu, Secretary of State Antony Blinken said at least 25 Americans had now been confirmed dead in Israel.

Netanyahu described some of the atrocities committed by Hamas.

"Hamas has shown itself to be an enemy of civilization: The massacring of young people in an outdoor music festival, the butchering of entire families, the murder of parents in front of their children and the murder of children in front of their parents. The burning of people alive, the beheadings," he said.

"Hamas should be treated exactly the way ISIS was treated," he said. "They shouldn't be spit out from the community of nations. No leader should meet them. No country should harbor them. And those that do should be sanctioned."

Blinken said he was speaking not only as an American diplomat, but as a Jew, a husband and a father of young children, saying it was impossible for him to look at photos of families killed "and not think of my own children."

"The same time that we've been shocked by the depravity of Hamas, we've also been inspired by the bravery of Israel citizens," he said. "The grandfather who drove over an hour to a kibbutz under siege, armed only with a pistol and rescued his kids and grandkids. The mother who died shielding her teenage son with her body, giving her life to save his -- giving him life for a second time."

Blinken remarked that some U.S. aid had already been delivered, and that more military aid was on his way. He said the administration would work with Congress to meet Israel’s additional needs and that bipartisan support for Israel was "overwhelming."

The secretary called on world leaders to condemn Hamas.

Blinken also reiterated that Israel "has the right -- indeed, the obligation -- to defend itself" but that it was "so important to take every possible precaution to avoid harming civilians."

-ABC News' Shannon Crawford