Israel launches massive airstrikes on downtown Gaza City, Netanyahu says: 'We have only started'

Israel's counteroffensive into Gaza extended through the night, including airstrikes in Gaza City, home to Hamas' centers of government.

Israeli warplanes pounded downtown Gaza City with relentless bombardments into early Tuesday, after Israel’s prime minister vowed retaliation against the Islamic militant group that would "reverberate for generations."

Israeli Prime Minister Benjamin Netanyahu gave a nationally televised address, where he said: "We have only started striking Hamas."

He added: "What we will do to our enemies in the coming days will reverberate with them for generations."

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Israel has, so far, issued warnings to civilians in areas of Gaza City and others where they intend to strike. The warnings have given civilians moments to evacuate so as to reduce the number of civilian deaths.

Hamas has threatened to start executing captured Israelis if strikes targeted civilians without warning. Israel said that Hamas is holding more than 150 soldiers and civilians hostage in Gaza.

The war is only expected to escalate as Israel could launch a ground invasion to eradicate Hamas from Gaza.

On Tuesday morning, Israel’s military said it regained effective control over areas near the Gaza Strip border with Israel, which was breached over the weekend in Saturday's surprise attack.

The Israeli military also said it recovered the bodies of roughly 1,500 Hamas militants on Israeli territory.

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The four-day-old war has already claimed at least 1,600 lives, after Hamas terrorists invaded the Gaza-Israel border and attacked towns and villages. 

Israel saw gun battles in the streets of its own towns for the first time in decades.

This is a developing story. Check back for updates.

The Associated Press contributed to this report.