The deadly Hamas militant attack on Israel and the massive retaliation it provoked from Jerusalem have thrust President Joe Biden into a Middle East crisis that risks expanding into a broader conflict and has left him fending off criticism from GOP presidential rivals that his administration's policies led to this moment.

The potential for prolonged and expanding violence could test Biden's leadership on both the world stage and at home as he tries to navigate between demonstrating unflinching support for Israel and fostering a broader peace in the combustible Mideast, where sympathetic militants were quick to loudly praise the action by Hamas.

Lebanon's Hezbollah group welcomed the attack as a response to "Israeli crimes." The group, which holds similar goals as Hamas for the destruction of the Israeli state, said its command was in contact with Hamas about the operation. A senior adviser to Iran's supreme leader also lauded the deadly attack. And Hamas said it was ready for a potentially long fight.

Gunfire, rockets and carnage: Israelis are stunned and shaken by unprecedented Hamas attack

Israeli media, citing rescue service officials, said at least 300 people were killed, including 26 soldiers, while in Gaza officials said 313 people had died. An Israeli military official said hundreds of militants had been killed and dozens captured.

Rear Adm. Daniel Hagari, an Israeli military official, said the military had killed 400 militants since the start of the war and captured dozens more.

Among the 313 killed in Gaza were 20 children, and close to 2,000 wounded, the Palestinian Health Ministry said.

Several 2024 Republican presidential contenders immediately tried to pin a portion of the blame on Biden. They sought to tie his recent decision to release $6 billion in blocked Iranian funds in exchange for freeing five Americans who had been detained in Iran to Saturday's complex attack by air, land and sea. The White House pushed back fiercely against the GOP criticism, noting that the money unfrozen last month in the prisoner swap has yet to be spent by Iran and can only be used for humanitarian needs.

5 things to know about the Hamas group's unprecedented attack on Israel

Iran has historically maintained strong ties with both Palestinian Hamas and Hezbollah.

A senior Biden administration official who briefed reporters on the condition of anonymity said it was "too early to say whether the state of Iran was directly involved in planning or supporting" the complex attack but noted Iran's deep ties to Hamas.

Biden and top aides spent Saturday consulting with European and Middle East leaders, including Israeli Prime Minister Benjamin Netanyahu. In remarks before reporters at the White House, Biden called the attacks "unconscionable" and pledged his administration would ensure Israel has "what it needs to defend itself."

"Let me say this as clearly as I can: This is not a moment for any party hostile to Israel to exploit these attacks to seek advantage," Biden said.

The attack only adds new complications as the Biden administration and Iran are locked in disputes over Tehran's nuclear program. Iran says the program is peaceful, but it now enriches uranium closer than ever to weapons-grade levels. Still, the administration hasn't given up hope on reviving a deal brokered during the Obama administration — and scrapped during the Trump White House — that eased sanctions on Iran in return for curbs to its nuclear program.

Israeli soldiers battle Hamas on second day of surprise attack; strikes level buildings in Gaza

Biden administration officials have also been working on brokering a normalization of relations between Israel and Saudi Arabia, the most powerful and wealthy Arab state. Such a deal has the potential to reshape the region and boost Israel's standing in historic ways.

But brokering such a deal was already seen as a heavy lift as the kingdom has said it won't officially recognize Israel before a resolution to the decades-long Israeli-Palestinian conflict. The new conflict adds an enormous new roadblock to Biden's ambitions, although the administration official said the White House did not see the Hamas attack derailing the effort.

Saudi Arabia's foreign ministry in a statement did not condemn the Hamas attack, but noted the kingdom's "repeated warnings of the dangers … of the situation as a result of the continued occupation, the deprivation of the Palestinian people of their legitimate rights, and the repetition of systematic provocations against its sanctities."

Netanyahu vowed in his own national address to avenge the startling attacks, pledging to "bring the fight to them with a might and scale that the enemy has not yet known."

Hamas fighters have taken an unknown number of civilians and soldiers captive into Gaza, in harrowing scenes posted on social media. Those images — and the mounting death toll — come 50 years and a day after invading forces from Egypt and Syria caught Israel by surprise with the launch of an attack on Israel that set off the 19-day conflict that became known as the Yom Kippur War.

Jonathan Schanzer, an analyst at the Washington think tank Foundation for Defense of Democracies, said Biden did "a good job" at keeping Israel's critics, particularly his fellow Democrats, at bay while Netanyahu sought to achieve his military objectives against Hamas during their last major conflict, an 11-day war in 2021. It will likely be tougher this time around.

"There will be mistakes that often happen on the battlefield, no military is perfect. That's when I think the president will come under fire from his left flank," Schanzer said.

Some in the 2024 Republican field were quick to place blame squarely on Biden for the Hamas assault.

Former President Donald Trump charged that the U.S. is perceived as being "weak and ineffective" on the global stage under Biden, opening the door to hostility against Israel. Florida Gov. Ron DeSantis accused Biden of "policies that have gone easy on Iran" and have "helped to fill their coffers." And South Carolina Sen. Tim Scott alleged the attack was "the Biden $6 billion ransom payment at work," a reference to the prisoner deal.

Biden administration officials pushed back against the argument.

"Let's be clear: the deal to bring U.S. citizens home from Iran has nothing to do with the horrific attack on Israel," said State Department spokesman Matthew Miller. "Not a penny has been spent, and when it is, it can only go for humanitarian needs like food and medicine."

Biden administration officials did not address whether Iran, in anticipation of using the money — now held in Qatari banks — for food, medicine, medical supplies and agricultural products, may have diverted other funds to Hamas or other proxies.

In a briefing with Senate Foreign Relations Committee staff on Saturday, administration officials said the U.S. had warned Iran "through interlocutors" that direct involvement in the Gaza situation would imperil any future initiatives the U.S. might consider with the Islamic Republic, according to a congressional aide familiar with the session.

Death toll from Israeli airstrikes in Gaza rises to 232

The officials did not elaborate on who the interlocutors were or what future initiatives would be in jeopardy, although acting deputy Secretary of State Victoria Nuland and the top U.S. diplomat for the Mideast, Barbara Leaf, both spoke to officials in Lebanon about the situation. Some Lebanese officials maintain contact with Iran, which supports the militant group Hezbollah in the country.

Another point of criticism leveled at the administration by Republicans is that its decision shortly after taking office to reverse a Trump-era ban on assistance to the Palestinians, including civilians in Gaza, may have helped fund the operation.

Administration officials roundly rejected this, saying their efforts to help Palestinian civilians in Gaza and elsewhere do not involve money that Hamas can use or divert.