Hamas has launched an unprecedented attack against Israel. Here's what to know

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A large-scale surprise attack by Gaza militants on Israel has left hundreds of Israelis dead, prompting vows of retribution from Israeli Prime Minister Benjamin Netanyahu and a lethal volley of retaliatory Israeli airstrikes.

"Citizens of Israel, we are at war - not in an operation, not in rounds - at war," Netanyahu said in a video message shortly after the militant group Hamas, which controls the coastal enclave of Gaza, fired a barrage of rockets and sent gunmen into Israel on Saturday in a multi-pronged and unprecedented attack in the long-running conflict.

By nightfall, Israeli air strikes on Gaza had killed hundreds of Palestinians and wounded over 1600, according to local health officials.

Still unknown are the fates of several hostages, including Israeli soldiers, that Hamas claims to have captured.

Here's what we know so far.

Around 6:30 a.m. local time on Saturday, when many Israelis were likely to have been asleep, sirens were heard as far north as the Tel Aviv area, east to Beer Sheba, and many other locations in between as rockets flew over Israel.

Militants from Gaza then entered Israeli territory by land, sea and air, with some using paragliders, the Israel Defense Forces (IDF) said. It warned Israelis who live near Gaza to stay in their homes.

Social media video obtained by CNN also showed multiple masked gunmen in a pickup truck driving down a street in the Israeli city of Sderot, which lies close to the Gaza border. Gunfire could also be heard in the video.

The IDF said around 2,200 rockets were fired at Israel. Hamas put the figure at 5,000. To put that in context, some 4,000 rockets were fired from Gaza into Israel during the 50-day war between the two sides in 2014.

Hamas military commander Muhammad Al-Deif called the operation "Al-Aqsa Storm" and said that the assault on Israel was a response to attacks on women, the desecration of the al-Aqsa mosque in Jerusalem and the ongoing siege of Gaza.

Hamas said it captured Israeli soldiers, posting videos of soldiers it allegedly captured on its social media accounts. Videos geolocated and authenticated by CNN suggested that at least one Israeli soldier was taken prisoner by the group.

"There are hostages and prisoners of war that Hamas took. There are also deaths among IDF soldiers. We don't have an exact number yet - we are at war," said IDF Spokesperson Rear Admiral Daniel Hagari.

The group also posted a video allegedly showing a captured Israeli military vehicle. A CNN journalist saw a military Humvee being paraded through Gaza. The IDF declined to comment on the matter. Another video circulating on social media showed a tank on fire near the Gaza border.

In response to the attack, Israel launched "Operation Swords of Iron," striking a number of targets in the Gaza strip. Netanyahu said Israel's response to the Hamas incursion will "exact a huge price" from the militant group.

"The IDF is reinforcing the south and the communities surrounding the Gaza Strip with several operational forces," Hagari said in a statement.

The IDF described the attack as "massive." Around noon local time, it said fighting was taking place on the ground in at least seven locations in the country.

IDF spokesperson Lt. Col. Richard Hecht told CNN that "tens of thousands" of Israeli reservists have also been called up to help, adding that he hopes "there will be no more live terrorists in Israel" by the end of the day.

He said Israel is "ready in the north" in case there are attacks from Lebanon or Syria.

Inside Gaza, the roar of Israeli warplanes could be heard followed by loud explosions and rising plumes of black smoke.

Palestinians were barred from crossing checkpoints into Jerusalem from the West Bank, and Israel said it will close all educational facilities on Sunday.

Tensions between Israel and the Palestinians have existed since before the nation's founding in 1948. Thousands of people on both sides have been killed and many more injured in the long-simmering conflict between the two sides over the past few decades.

Violence has been particularly heightened this year. The number of Palestinians - militants and civilians - killed in the occupied West Bank by Israeli forces is at its highest in nearly two decades. The same is true of Israelis and foreigners - most of them civilians - killed in Palestinian attacks.

Israel and Hamas have been involved in armed conflict dating back as early as the 1987 First Palestinian Intifada, or uprising, against Israel's occupation of the Gaza Strip and the West Bank.

Israel captured Gaza from Egypt in a 1967 war, then withdrew in 2005. The territory, home to some 2 million Palestinians, fell under Hamas' control in 2007 after a brief civil war with Fatah, a rival Palestinian faction that is the backbone of the Palestinian Authority.

After Hamas seized control of Gaza, Israel and Egypt imposed a strict siege on the territory, which is ongoing. Israel also maintains an air and naval blockade on Gaza.

Before Saturday's operation, the last war between Hamas and Israel was in 2021, which lasted for 11 days and killed at least 250 people in Gaza and 13 in Israel.

Saturday's assault occurred on the 50th anniversary of the 1973 war, when Israel's Arab neighbors launched a surprise attack on Israel on Yom Kippur, the holiest day in the Jewish calendar, on October 6, 1973.

Hamas is an Islamist organization with a military wing that came into being in 1987, emerging out of the Muslim Brotherhood, a Sunni Islamist group that was founded in the late 1920s in Egypt.

The word "Hamas" is itself an acronym for "Harakat Al-Muqawama Al-Islamia" - Arabic for Islamic Resistance Movement. the group, like most Palestinian factions and political parties, insists that Israel is an occupying power and that it is trying to liberate the Palestinian territories. It considers Israel an illegitimate state.

Its refusal to recognize Israel is one reason why it has rejected peace talks in the past. In 1993, it opposed the Oslo Accords, a peace pact between Israel and the Palestine Liberation Organization (PLO).

The group presents itself as an alternative to the Palestinian Authority (PA), which has recognized Israel and has engaged in multiple failed peace initiatives with it. The PA is today led by Mahmoud Abbas and is based in the Israeli-occupied West Bank.

Hamas has over the years claimed many attacks on Israel and has been designated as a terrorist organization by the United States, the European Union and Israel. Israel accuses its archenemy Iran of backing Hamas.

The al-Aqsa compound is one of the most revered places in Islam and Judaism. The sacred grounds, known to Muslims as Al Haram Al Sharif (Noble Sanctuary) and to Jews as Temple Mount have been a flashpoint of tensions between Israel and the Palestinians for decades.

Hamas says it launched Saturday's "Al-Aqsa Storm" attack in part to defend the holy site.

Only Muslims are allowed to pray in the compound under a status quo arrangement originally reached more than a century ago. Non-Muslim visitors are allowed visits at certain times and only to certain areas of the complex.

But many in the Muslims world fear that the right to be the sole worshipers there has been eroded and that the sites themselves are being threatened by a growing far-right Jewish movement and Israel's far-right government.

Clashes have frequently broken out at the site between Palestinian worshipers and Israeli forces. Police raided the compound several times over the last year.

The complex lies in East Jerusalem, which the Palestinians want as the capital of their future state and which most of the international community considers to be occupied territory. Israel captured it from Jordan in its six-day war in 1967 and considers both East and West Jerusalem as its united, "eternal capital."

Western states condemned the Hamas attack and pledged support for Israel while Arab states, including those that have recognized Israel, called for calm.

United States President Joe Biden spoke to Israeli Prime Minister Benjamin Netanyahu on Saturday and said that Washington "unequivocally condemns this appalling assault against Israel by Hamas terrorists from Gaza."

"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," he said.

Saudi Arabia, which is holding talks with the US to potentially normalize relations with Israel, said it is following closely the "unprecedented" situation and called on "both sides to immediately stop the escalation," according to a statement by the Saudi foreign ministry on X.

The UAE, which recognized Israel in 2020, called for "the exercise of maximum restraint and an immediate ceasefire to avoid dangerous repercussions."

Israel's adversaries welcomed the attack. Major General Rahim Safavi, an adviser to Iran's Supreme Leader Ayatollah Ali Khamenei, said: "We congratulate the Palestinian fighters and will stand by their side until the liberation of Palestine and holy Jerusalem," state-run ISNA said.

Hezbollah, the Iran-backed Lebanese militant group, praised the assaults and said it is in contact with Palestinian militant groups "at home and abroad," its Al Manar channel said.

CNN's Mostafa Salem and Hande Atay Alam contributed to this report