Israel began a major military operation in the Gaza Strip on Saturday in response to a surprise attack by Hamas, which left hundreds dead in the bloodiest day of conflict in years and has triggered the most serious escalation in the Israeli-Palestinian conflict in years.

In Gaza overnight, the Israeli military said it struck over 400 Hamas and Islamic Jihad targets including weapons manufacturing sites, buildings used by militant leaders and underground tunnels. Israel is at war and we will extract an unprecedented price from Hamas, Prime Minister Benjamin Netanyahu said, after declaring the attack on Thursday constituting an ‘attack on us’.

We are entering on to a long and difficult war  forced on us by a murderous Hamas attack, Netanyahu said during a televised address. This time, of course, the first stage to the end is the destruction of a part of the entire enemy forces that broke into our territory, while we’re also entering an offensive stage.

Hamas militants fired thousands of rockets into southern Israel and armed gunmen across the border in what went on to become a multi pronged assault that Hamas launched on Saturday morning before the onset of the military offensive. Israels towns and military bases near the Gaza Strip were attacked on a major Jewish holiday when militants infiltrated several towns.

The attack was catalogued as Israel’s deadliest in decades, killing at least 250 Israelis and more than 1,500 others, Israeli officials said. In the Gaza strip, Hamas fighters paraded smuggled some Israeli soldiers and civilians around the streets and hostage took an unknown number to remain unknown.

In Israel, Israeli airstrikes now have killed at least 232 Palestinians and wounded nearly 1,700, the Palestinian Health Ministry in Gaza said Sunday morning. Fighting continues and an expected rising death toll on both sides.

Violence has broken out all over the region and far beyond. The United States strongly condemned the Hamas attack and vowed full support for Israel’s right to defend itself. Netanyahu spoke with President Joe Biden and he told the Israeli leader that we stand ready to offer all appropriate means of support to the people of Israel, and that is what we have done, Biden said.

Iran and some Arab states blamed Israeli policies for the escalation while other Western nations joined with Israel in voicing support. On Sunday, UN Security Council met emergency to discuss the crisis.

It stunned many observers, and raised questions about intelligence failures on the Israeli side at a scale and sophistication never seen before. But the militant group, which has run Gaza since 2007, has carefully planned the assault to coincide with the Jewish Sabbath and holiday of Simchat Torah.

Israel mobilised hundreds of thousands of reservists, and troops were moved near the Gaza border, generating talk of an impending ground invasion of the territory. Consequently, such an operation would be nearly sure to result in heavy casualties on both sides.

The pace of escalation risks rolling back recent efforts to normalise relations between Israel and Saudi Arabia, among other diplomatic progress in the region. It too is a major test for Netanyahu’s far-right government, which has ploughed hard line against the Palestinians.

The renewed fighting brings fresh fears of another devastating war for residents of Gaza, an impoverished coastal enclave of 2 million people. Intense destructive conflict has led to widespread destruction and civilian deaths in the densely populated area in the past.

Activist Ahmed Abu Artema from Gaza said We are scared and we don’t know what is about to happen. There’s also that feeling that something had to give after a lifetime of blockade and hopelessness.”

There were fears the violence could spread to other fronts as the two sides dug in for what could be a drawn-out conflict. Israel exchanged fire with Hezbollah militants in Lebanon on Sunday, and fears are mounting of a wider regional conflagration.

The coming days will hold a key to whether rising tensions can be deescalated or whether the region is in a course for a heavy war with broad implications.