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.

The death toll continues to climb with over 300 people reported dead in Gaza and Israel.

The Palestinian Health Authority said 198 are dead and 1,610 others are injured in Gaza. According to the Israeli Health Ministry over 100 people are dead and over 900 others are injured.

Just before sundown in a western Gaza City, a massive explosion and fireball were reported after Israel warplanes hit a high-rise apartment complex. The Gaza Interior Ministry said the building housed approximately 100 families.