How will America respond to the attack against Israel?

With thousands of casualties resulting from rockets being fired from Gaza into Israel by Hamas militants, the United States is crafting a response to what President Joe Biden termed "the appalling Hamas terrorist attacks in Israel."

Biden delivered an address Saturday afternoon condemning the "unconscionable" actions by Hamas as the U.S. reactions continue to unfold to the attack, which coincided with the 50th anniversary of the Yom Kippur War.

Biden administration responds

The White House condemned the attacks on Saturday.

"In this moment of tragedy, I want to say to [Hamas] and to the world and to terrorists everywhere that the United States stands with Israel. We will not ever fail to have their back," Biden said from the White House Saturday afternoon.

The president added that "Israel has the right to defend itself and its people, full stop" and his administration's support for Israel's security is "rock solid."

Earlier Saturday, Biden confirmed in a written statement that he had spoken with Israeli Prime Minister Benjamin Netanyahu.

"The United States unequivocally condemns this appalling assault against Israel by Hamas terrorists from Gaza, and I made clear to Prime Minister Netanyahu that we stand ready to offer all appropriate means of support to the Government and people of Israel," Biden's statement said.

Secretary of State Antony Blinken echoed the president's statement, saying that the U.S. "unequivocally condemns the appalling attacks by Hamas terrorists against Israel, including civilians and civilian communities."

"We stand in solidarity with the government and people of Israel, and extend our condolences for the Israeli lives lost in these attacks. We will remain in close contact with our Israeli partners. The United States supports Israel’s right to defend itself," he added.

Pentagon press secretary Brig. Gen. Pat Ryder confirmed that Defense Secretary Lloyd Austin has spoken with Israeli Minister of Defense Yoav Gallant "to convey his condolences for the victims of this appalling, abhorrent terrorist attack by Hamas on Israel."

"He conveyed that his thoughts are with the families of the victims, and that he sends his best wishes for recovery to those who were injured," Ryder said.

The Biden administration has said that Israel has a right to defend itself, though it is unclear how large a retaliatory strike Israel could execute before the U.S. would express disapproval.

The U.S. Office of Palestinian Affairs tweeted and then deleted a message urging "all sides to refrain from violence and retaliatory attacks."

"We unequivocally condemn the attack of Hamas terrorists and the loss of life that has incurred. We urge all sides to refrain from violence and retaliatory attacks. Terror and violence solve nothing," the message said, multiple outlets reported.

The State Department said the deleted tweet was not approved.

"This tweet was not approved and does not represent U.S. policy. The United States condemns the terrorist attack against Israel and unequivocally supports Israel’s right to defend itself, as the President and Secretary have both made clear," State Department Spokesperson Matthew Miller said in a statement.

Biden takes domestic political fire

Republican presidential candidates Saturday morning were quick to blame the Biden administration for the attack on Israel.

Specially, they point to the fact that last month, the U.S. agreed to un-freeze $6 billion in Iranian funds as a part of a prisoner exchange.

"I say to you with a heavy heart. This is what happens when you have a president like Joe Biden, who spends the last two and a half years projecting weakness on the world stage for from that disastrous withdrawal in Afghanistan, to kowtow into the mullahs in Iran for the last two and a half years begging them to get back in the Iran nuclear deal," former Vice President Mike Pence said in Iowa on Saturday.

"First, I just want to say that today, America stands with Israel against the terrorist attacks being perpetuated by Hamas, who is backed by Iran, and unfortunately funded by this American president with what he's done to Iran," Florida Gov. Ron DeSantis said at a campaign event in Iowa. "It's a disgrace."

Asked how he thought the Biden administration should respond to the attack, DeSantis first asked reporters whether Biden had made a statement yet on what happened. When the group responded yes, the governor said Biden needed to speak. (Biden had not yet delivered remarks from the White House.)

"This is a time for moral clarity. This is a time for the President of the United States to say unequivocally we stand with the people of Israel. We stand with Prime Minister Netanyahu. I know [Biden] doesn't always get along with and say you cannot have terrorists, raining down rockets on your civilian population going into these communities and killing people and hand to hand situations. That is totally unacceptable. And so, we've got to be very clear on that as Americans and he needs to make sure that he is doing moral clarity," DeSantis said.

White House Deputy Press Secretary Andrew Bates called the claims that Biden funded the attack "a shameful lie in every respect."

"Not a cent has been spent. They won't go to the Iranian govt. They can only be used for verifiable purchases of humanitarian needs like food & medicine," Bates added.

An intelligence failure?

There may have been indicators that intelligence should have picked up that an attack would happen in Israel, according to Mick Mulroy, former deputy, assistant secretary, defense for the Middle East, retired CIA, paramilitary officer, retired US Marine, and an ABC News national security and defense analyst. It was the start date of a similar attack in 1973 that started the Yom Kippur War, and there were likely indications of the buildup of munitions and the preparation of the assault force, and there was cyber activity in Israel prior to the assault, Mulroy noted.

The attack itself was very complex. It was preceded by a rocket and missile barrage of thousands, which likely buttoned up the IDF and a lot of the police force in Israel; it was then followed by denial of service attacks in Israel and then an air, sea, and land attack with assaulters coming in on tractors to breach firms technical pickup trucks with soldiers, small boats, and paragliders. These multiple means of attack probably gave the IDF a significant tactical dilemma, per Mulroy.

The next phase will be mostly up to Israel, likely a ground invasion of Gaza to capture or kill militants and destroy the weapons and munitions supply, of which the Palestinians or prey pay the worst price, according to Mulroy who said this will be followed likely by special forces operations in an attempt to decapitate Hamas leadership and recover hostages and airstrikes as well.

The operation's complexity indicates a nation-state like Iran was behind this supporting the attackers with weapons and munitions, intelligence, and possibly operational planning. If Iran was behind it and is orchestrating this in any sense, they have multiple proxies around Israel and could escalate this by initiating attacks from multiple directions, including Syria, according to Mulroy.

Another possible consequence of the attack is the dissolution of the current U.S.-brokered negotiations between Israel and Saudi Arabia.

At least 300 people have died and over 2,000 have been injured so far in Gaza, according to Israeli authorities.

ABC News' Mick Mulroy, Shannon Crawford, Hannah Demissie, MaryAlice Parks, Abby Cruz and Carly Roman contributed to this report.