At least 600 people in Israel have been killed in the surprise attacks by Hamas and at least 370 have been killed in retaliatory strikes in Gaza, Israeli and Palestinian health officials said Sunday.

At least 2,156 people have been injured in Israel, according to the Israeli Health Ministry. At least, 2,200 have been injured in Gaza, the Palestinian Health Authority said.

-ABC News' Joseph Simonetti

Republican presidential candidate Chris Christie slammed the dysfunctionin> the U.S. House of Representatives, where a small group of Republican lawmakers last week voted along with the Democratic minority to oust Speaker Kevin McCarthy, for leaving it without a way to pass legislation, including foreign aid to Israel.

"What I would be doing is making sure, one, that Israel has everything that it needs to be able to take whatever actions it needs to take. And this is the problem with not having a speaker right now," Christie, the former New Jersey governor, told ABC News' "This Week" anchor George Stephanopoulos.

Christie, a former ABC News contributor, added, "The actions taken by some members of my party were wholly irresponsible without this going on. They're now even putting a brighter light on the irresponsibility of not having someone in place."

Christie also said his top priorities if he were president would be engaging allies in the region, such as Jordan and Saudi Arabia, to keep the conflict from spreading and to serve as a "sounding board" to Israeli Prime Minister Benjamin Netanyahu to help him "think through the ramifications of every step they're going to take to defend themselves and to try to do the best they can to eliminate the leadership of Hamas."

-ABC Nicholas Kerr

For the first time since Saturday's surprise attack by Hamas militants, the Israeli government confirmed that a number of people, including civilians, have been taken hostage.

Israel's Government Press Office said Sunday that at least 100 Israeli citizens and soldiers are being held hostage by Hamas fighters.

U.S. Secretary of State Antony Blinken said in separate interviews on CNN and NBC News that there is a possibility Americans are among those killed or taken hostage and that the U.S. Department of State is investigating.

-ABC News' Joe Simonetti

Secretary of State Antony Blinken on Sunday pushed back on growing Republican criticism that the White House's decision to allow the transfer of some $6 billion of Iran's oil revenue, in exchange for the release of five American detainees, was a sign of "appeasement" ahead of the surprise attack on Israel by Hamas militants.

"Not a single dollar from that account has been spent to date and, in any event, is very closely and carefully regulated by the Treasury Department," Blinken told ABC's "This Week" anchor George Stephanopoulos.

Blinken stressed that the funds are designated solely for humanitarian purposes and are not U.S. tax dollars.

-ABC News' Adam Carlson

The United States is warning against other countries from becoming involved in the Israel-Gaza conflict amid fears that it could spread into a wider war in the Middle East.

"This is not the moment for other parties who are hostile to Israel to seek advantage or to seek to exploit the attacks that have taken place," Jon Finer, deputy national security adviser to U.S. President Joe Biden, Monday on "Good Morning America."

"That is part of why the United States has moved the Carrier Strike Group to the eastern Mediterranean to send a strong and unmistakeable signal that no one else should get involved in this," he added. "We'll see how things unfold in the course of the coming days."

Finer also emphasized that the U.S. believes "Israel has every right to defend itself full-stop."

"Israel will ultimately make the decisions about how it chooses to go about and conduct that defense," he said. "We are offering support in a number of ways."

Finer noted that "more U.S. steps to show support and solidarity for Israel" can be expected.

"The U.S. is going to offer support at every level for Israel's defense and we are working through details those details with our intelligence professionals, our military and our diplomats," he added. "We are in daily -- in fact, many times a day -- contact with Israeli counterparts to see what they need and offer it."

The U.S. "strongly suspect[s] that there will be American citizens among those killed" in the ongoing conflict, according to Finer.

"We are looking obviously very intensively into whether there were any Americans were among those who have been abducted and brought to Gaza. This is still ongoing," he said. "There is still fighting inside Israel as we speak and there is a bit of fog of war in terms of the ability to gather specific information. And so, we will have much more to say about this at the right time."

When asked for comment on a recent report by The Wall Street Journal that Iran was behind Hamas' latest attack on Israel, Finer responded: "We have no direct information to confirm that report. We've obviously seen it, we're looking into it, but we do not have the ability to corroborate it at this time."

"What we can be quite clear about is that Iran is broadly complicit in these attacks for having supported Hamas going back decades -- for having provided financial support, for having provided training, for having provided weapons to Hamas," he added. "What we don't have is direct information that shows Iranian involvement in ordering or planning the attacks that took place over the last couple days. It's something that we’re going to keep looking at closely."