Over 970 people have died in a large-scale attack originating from the Gaza Strip by Palestinian militant group Hamas and retaliatory strikes it provoked from Israel, local authorities and media reported Sunday.

At least 600 people have been killed in Israel since Hamas launched thousands of rockets by surprise early Saturday morning attack, local broadcaster Channel 12 reported, while more than 370 Palestinians have been killed by Israeli air strikes on Gaza, according to the Palestinian Health Ministry.

In addition to the rocket barrage, Hamas gunmen crossed from Gaza into as many as 22 locations inside Israel, shooting civilians and security forces and taking Israeli hostages as they went, according to The Associated Press.

Fire and smoke rise following an Israeli airstrike in Gaza City, on Oct. 7, 2023. (AP/Kyodo)

Israel's Prime Minister Benjamin Netanyahu said Saturday his country is "at war" following Hamas' attack, considered the worst on the country in recent history, while his office approved steps to destroy the group's military and governmental capabilities.

"We will take mighty vengeance for this black day," Netanyahu said, vowing to continue "without reservation and without respite."

Israel launched retaliatory air strikes on targets in Gaza.

U.S. President Joe Biden said in a Saturday address that "the United States stands with Israel."

A Palestinian man sits in the rubble of a building after it was struck by an Israeli airstrike, in Gaza City, on Oct. 8, 2023. (AP/Kyodo)

Responding to Hamas' attack, Japanese Prime Minister Fumio Kishida on Sunday said, "Japan strongly condemns the attack which severely harmed innocent civilians. I express my condolences to the bereaved families and heartfelt sympathies to the injured."

Kishida also expressed concern for those killed and wounded in the retaliatory attacks in Gaza.

The U.N. Security Council is scheduled to hold a closed-door emergency meeting on the Hamas attack on Sunday.

The conflict is likely to become the worst since 2014 -- when around 2,250 people in Gaza and about 70 in Israel were killed in a seven-week war -- if Israel moves into a ground invasion.