Israel vowed to escalate its response to an attack by the Palestinian militant group Hamas with a ground offensive, while U.S. President Joe Biden pledged support for Israel and issued a warning to anyone who might seek to take advantage of the situation.

Israel said dozens of its fighter jets struck more than 200 targets overnight on Wednesday in a neighbourhood of Gaza City that it said had been used by Hamas to launch its unprecedented wave of attacks.

Gaza’s health ministry said at least 950 people have been killed and 5,000 injured in the crowded coastal enclave.

On Saturday, Hamas gunmen from the Gaza Strip rampaged through parts of southern Israel, in the deadliest Palestinian militant attack in Israel’s history.

Israel’s military said the death toll in Israel had reached 1,200 and more than 2,700 people had been wounded.

“We have sustained extremely heavy casualties,” military spokesperson Lieutenant Colonel Jonathan Conricus said in a video briefing on X, formerly known as Twitter.

The victims were overwhelmingly civilians, gunned down in homes, on streets or at an outdoor dance party. Scores of Israelis and others from abroad were captured and taken to Gaza as hostages, some shown on social media being paraded through the streets.

Hamas militants holding Israeli soldiers and civilians hostage on Monday threatened to execute a captive for each home in Gaza hit without warning, but as night fell on Tuesday there was no indication they had done so.

Israeli Defence Minister Yoav Gallant, speaking to soldiers near the Gaza fence, said: “Hamas wanted a change and it will get one. What was in Gaza will no longer be.”

“We started the offensive from the air, later on we will also come from the ground. We’ve been controlling the area since Day 2 and we are on the offensive. It will only intensify.”

At least 1,000 gunmen who had infiltrated from Gaza had been killed, the Israel Hayom newspaper reported.

Israel withdrew troops from Gaza in 2005 after 38 years of occupation, and has kept it under blockade since Hamas seized power there in 2007. The siege it announced on Monday would keep out food and fuel.

On Israel’s northern border, a salvo of rockets was fired from southern Lebanon towards Israel, prompting Israeli shelling in return, three security sources said.

More shells launched from Syrian territory landed in open areas in Israel, further raising fears that the violence could lead to a wider war.

“We do not yet know if these rockets were fired by the Syrian armed forces, by any of the many Iranian militias that exist and are welcomed by the Syrian regime, or Hezbollah or any other action,” said Israel’s Lieutenant Colonel Conricus.

“What we do know is that we retaliated fire toward the sources of fire, and currently the situation there is quiet.”

Sirens warning of incoming rocket fire blared overnight in Israeli communities near the Gaza border.