Hundreds dead, thousands injured in Israel and Gaza after Hamas launches rockets; Israel declares war

Hundreds of people in Israel are reported dead and thousands are injured after Hamas militants fired rockets from Gaza and launched a ground incursion, Israeli officials said Saturday. Hundreds more are dead and injured in Gaza after Israel launched retaliatory attacks, Palestinian officials reported.

Up to 50 Israelis had been held hostage by militants on Saturday, the Israeli Command told ABC News. They were freed Saturday evening, but officials said an unknown number could still be held in captivity. There is still ongoing fighting in several towns, the officials said.

"We are at war. We will win," Israeli Prime Minister Benjamin Netanyahu said on Saturday.

The Israeli Defense Forces earlier declared "a state of alert for war," according to a statement issued by the IDF.

According to the IDF, a high-ranking Israel Defense Forces commander was killed in action Saturday.

Col. Jonathan Steinberg, 42, the commander of the Nahal Brigade, was killed during a confrontation with Hamas, IDF said.

Palestinian militants fired at least 2,200 rockets toward Israel, the IDF said. Meanwhile, Hamas claimed at least 5,000 rockets were fired, all landing in southern and central Israel.

Air raid sirens began sounding in Jerusalem at 8:15 a.m. local time warning citizens of the attack.

"Over the past hour, the Hamas terrorist organization launched massive barrages of rockets from Gaza into Israel, and its terrorist operatives have infiltrated into Israel in a number of different locations in the south," the IDF said early Saturday.

"Civilians in southern and central Israel must remain near shelters, and in the area surrounding the Gaza Strip, inside shelters," the IDF continued. "The Chief of the General staff is currently conducting a situational assessment and approving plans for the IDF's continued activity. The Hamas terrorist organization is responsible for these attacks and will face consequences for them."

Netanyahu released a video statement Saturday, making his first comments in Hebrew.

"This morning, Hamas launched a murderous surprise attack against the State of Israel and its citizens. We have been in this since the early morning hours. I convened the heads of the security establishment and ordered – first of all – to clear out the communities that have been infiltrated by terrorists," Netanyahu said. "This currently is being carried out. At the same time, I have ordered an extensive mobilization of reserves and that we return fire of a magnitude that the enemy has not known. The enemy will pay an unprecedented price. In the meantime, I call on the citizens of Israel to strictly adhere to the directives of the IDF and Home Front Command. We are at war and we will win it."

Elsewhere, the IDF confirmed that they are striking Hamas targets in Gaza as a result of the incursion, according to a statement on social media.

In remarks from the White House on Saturday, President Joe Biden said U.S. support for Israel is "rock solid and unwavering."

"Israel has the right to defend itself and its people full stop. There is never a justification for terrorist attacks and my administration's support for Israel's security is rock solid and unwavering. 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. The world is watching," Biden said.

In a statement earlier, Biden said he spoke with Netanyahu, telling him that the U.S. "condemns" Hamas' assault on Israel.

"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," the statement said.

The U.S. embassy in Israel earlier said that it is "closely monitoring" the security situation.

"The U.S. Embassy is aware that there have been casualties as a result of these incidents. U.S. citizens are reminded to remain vigilant and take appropriate steps to increase their security awareness as security incidents, including mortar and rocket fire, often take place without warning," the embassy said in the statement announcing the security alert.

U.S. Embassy personnel are currently sheltering in place and the ambassador condemned the attack in a message on social media, saying "I condemn the indiscriminate rocket fire by Hamas terrorists against Israeli civilians. I am in contact with Israeli officials, and fully support Israel's right to defend itself from such terrorist acts."

"The United States unequivocally condemns the unprovoked attacks by Hamas terrorists against Israeli civilians. There is never any justification for terrorism," read a statement from National Security Council spokesperson Adrienne Watson, condemning the attacks against Israel. "We stand firmly with the Government and people of Israel and extend our condolences for the Israeli lives lost in these attacks. National Security Advisor Jake Sullivan has spoken to Israeli National Security Advisor Tzachi Hanegbi and we remain in close contact with our Israeli partners."

Secretary of Defense Lloyd Austin in a statement said he is 'closely monitoring" the situation and said the Department of Defense "will work to ensure that Israel has what it needs to defend itself and protect civilians from indiscriminate violence and terrorism."

A U.S. defense official told ABC News that Austin had a call with his team this morning including U.S. Centcom Commander Gen. Eric Kurilla – Israel falls under Centcom's area of responsibility.

U.S. Secretary of State Anthony Blinken said the U.S. supports Israel's right to defend itself and "unequivocally condemns the appalling attacks by Hamas terrorists against Israel, including civilians and civilian communities."

Palestinian President Mahmoud Abbas says his people have the right to defend themselves against the "terror of settlers and occupation troops," according to Reuters.

Today marks the day that 50 years ago in 1973 when the Yom Kippur War -- or the Arab/Israeli War -- began.

ABC News' Joseph Simonetti contributed to this report.