JERUSALEM, Oct 11 (Reuters) - 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.

The Israeli military said dozens of its fighter jets struck more than 200 targets overnight 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 900 people have been killed and 4,600 wounded 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.

Israeli public broadcaster Kan reported the weekend death toll had reached 1,200.

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."

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, prompting Israel to return fire, the military said, further raising fears that the violence could lead to a wider war.

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

At the White House, Biden called the Hamas attacks "an act of sheer evil" and said Washington was rushing additional military assistance to Israel, including ammunition and interceptors to replenish the Iron Dome aerial defense system. He called for Israel to follow the "law of war" in its response.

He told reporters the United States had "enhanced our military force posture in the region to strengthen our deterrence," including by moving an aircraft carrier strike group and fighter aircraft.

"Let me say again to any country, any organization, anyone thinking of taking advantage of the situation, I have one word: don’t," said Biden, in an apparent reference to Iran and its proxies in the region.

U.S. officials say they do not have evidence Iran orchestrated the attacks, but point to Iran's long-term support for Hamas.

Biden also despatched his top diplomat Antony Blinken to Israel, who will deliver "a message of solidarity and support," State Department spokesperson Matthew Miller said.

Israeli Prime Minister Benjamin Netanyahu's right wing coalition and opposition leaders were close to forming an emergency unity government.

A meeting between Netanyahu and former Defence Minister Benny Gantz was scheduled for Tuesday but was delayed to Wednesday.

Photo from AFP