Israel says it is conducting a "complete siege" on the Gaza Strip, one of two Palestinian-held areas within Israel. Rockets used in the weekend's attacks were believed to come from the Gaza Strip.
Israel sealed off the Gaza Strip, cutting off access to food and fuel. Power was cut during the weekend, days earlier.
On Monday senior Defense Department officials said the U.S. had started deliveries of munitions and equipment to Israel.
President Biden said in a statement that 11 Americans had been killed in the violence.
"The safety of American citizens — whether at home or abroad — is my top priority as President," his statement read. "While we are still working to confirm, we believe it is likely that American citizens may be among those being held by Hamas. I have directed my team to work with their Israeli counterparts on every aspect of the hostage crisis, including sharing intelligence and deploying experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts."
Israel Defense Spokesperson Jonathan Conricus confirmed 700 Israelis have been killed following a series of attacks throughout the country over the weekend by the Palestinian militant group Hamas.
Conricus estimated that over 2,000 were wounded, many of them critically.
The Palestinian Health Ministry has confirmed 493 fatalities.
Videos shared on social media showed rockets striking a music festival and other civilian areas.
Conricus said that more than 1,000 members of Hamas participated in the attack. Late Sunday, Conricus said Israeli forces were working to get rid of remaining Hamas fighters.
“We assessed that there were approximately 1,000 terrorists who participated in yesterday’s invasion of Israel, about 1,000 bloodthirsty Palestinians who went house to house and building to building in search of Israeli civilians,” Conricus said. “It is by far the worst day in Israeli history. Never before have so many Israelis been killed by one single thing, let alone enemy activity, in one day.”
"If you're American and want to compare this to something in American history, this could be a 9/11 and a Pearl Harbor wrapped into one."
What is Hamas? The Palestinian group behind the Israel attack
The Israeli military has accused Hamas militants of behaving like the Islamic State group as fighting between the two sides continues.
The Israel Defense Forces continued sounding sirens on Monday, suggesting the threat of attacks are continuing. Nearly 300,000 Israelis have been called up as reserves.
The United States government considers Israel its strongest ally in the Middle East and Hamas as a terrorist organization. On Sunday, Secretary of State Antony Blinken called a number of Middle Eastern leaders to rally support for Israel.
Hamas has backing among many Palestinians, who are increasingly fed up with Israel’s decades-long military occupation of land. Hamas is also backed by some Middle Eastern leaders, including Iran, which provides financial assistance and military support through weapons and training.
Among the leaders Blinken spoke with was Palestinian Authority President Mahmoud Abbas, whom he called upon to condemn the weekend's attacks.
